chore: prlinter not triggered when "Codebuild PR Build" completes (#36057)

Added steps to save and upload pull request information.

### Issue # (if applicable)

Closes #36055

### Reason for this change

THE PROBLEM:
------------
PR Linter can't determine which PR to validate because:
  1. No pr_info artifact exists
  2. pull_requests array is empty (GitHub limitation)


THE SOLUTION:
-------------
Add these steps to codebuild-pr-build.yml:

```yaml
  - name: Save PR info for PR Linter
    if: github.event_name == 'pull_request'
    run: |
      mkdir -p ./pr
      echo "${{ github.event.pull_request.number }}" > ./pr/pr_number
      echo "${{ github.event.pull_request.head.sha }}" > ./pr/pr_sha

  - name: Upload PR info artifact
    if: github.event_name == 'pull_request'
    uses: actions/upload-artifact@v5
    with:
      name: pr_info
      path: pr/
```

WHY IT WORKS:
-------------

```
BEFORE (BROKEN):
┌─────────────────────────────────────────────────────────┐
│ Codebuild PR Build                                      │
│   Context: github.event.pull_request.number = 36049 ✓   │
│   Context: github.event.pull_request.head.sha = fc4... ✓│
│                                                          │
│   ✗ Does NOT save this info                             │
│   ✗ Does NOT upload artifact                            │
└─────────────────────────────────────────────────────────┘
                    │
                    │ workflow_run event
                    ▼
┌─────────────────────────────────────────────────────────┐
│ PR Linter (workflow_run context)                        │
│   Context: github.event.workflow_run.pull_requests = [] │
│   Context: No PR info available ✗                       │
│                                                          │
│   Tries to download pr_info artifact ──► ✗ Not found   │
│   Tries pull_requests[0].number ──────► ✗ Empty array  │
│                                                          │
│   Result: ✗ CANNOT DETERMINE PR NUMBER                  │
└─────────────────────────────────────────────────────────┘


AFTER (FIXED):
┌─────────────────────────────────────────────────────────┐
│ Codebuild PR Build                                      │
│   Context: github.event.pull_request.number = 36049 ✓   │
│   Context: github.event.pull_request.head.sha = fc4... ✓│
│                                                          │
│   ✓ Saves PR number to file: pr/pr_number               │
│   ✓ Saves PR SHA to file: pr/pr_sha                     │
│   ✓ Uploads pr_info artifact                            │
│      └─ Contains: pr_number=36049, pr_sha=fc4...        │
└─────────────────────────────────────────────────────────┘
                    │
                    │ workflow_run event
                    │ (artifact travels with event)
                    ▼
┌─────────────────────────────────────────────────────────┐
│ PR Linter (workflow_run context)                        │
│   Context: github.event.workflow_run.pull_requests = [] │
│                                                          │
│   Downloads pr_info artifact ─────────► ✓ Found!       │
│     └─ Extracts pr/pr_number ────────► ✓ 36049         │
│     └─ Extracts pr/pr_sha ───────────► ✓ fc4...        │
│                                                          │
│   Result: ✓ PR NUMBER DETERMINED = 36049                │
│           ✓ Can now validate the correct PR             │
└─────────────────────────────────────────────────────────┘
```

KEY INSIGHT:
------------

```
┌──────────────────────────────────────────────────────────┐
│  The PR context EXISTS in Codebuild PR Build             │
│  (triggered by pull_request event)                       │
│                                                           │
│  But it DISAPPEARS in PR Linter                          │
│  (triggered by workflow_run event)                       │
│                                                           │
│  Solution: PERSIST the context via artifact              │
│            (artifacts survive the context switch)        │
└──────────────────────────────────────────────────────────┘
```

CONTEXT AVAILABILITY:
---------------------

Codebuild PR Build (pull_request trigger):
  ✓ github.event.pull_request.number
  ✓ github.event.pull_request.head.sha
  ✓ github.event.pull_request.* (all PR data)

PR Linter (workflow_run trigger):
  ✗ github.event.pull_request (doesn't exist)
  ✗ github.event.workflow_run.pull_requests (empty array)
  ✓ github.event.workflow_run.id (can download artifacts)


ARTIFACT AS BRIDGE:
-------------------

```
  Codebuild Context          Artifact           PR Linter Context
  ─────────────────          ────────           ─────────────────

  PR #36049                                     workflow_run event
  pull_request event                            (no PR context)
       │                                              │
       │ Has PR data                                  │ No PR data
       │                                              │
       ▼                                              ▼
  Save to files          ──────────►            Download files
  - pr_number: 36049        Upload               - pr_number: 36049
  - pr_sha: fc4...          artifact             - pr_sha: fc4...
                            pr_info
                                                 ✓ PR context restored!
```

EXECUTION FLOW WITH FIX:
-------------------------

```
1. PR opened ──► Codebuild PR Build starts
                 │
                 ├─ github.event.pull_request.number = 36049 ✓
                 ├─ github.event.pull_request.head.sha = fc4... ✓
                 │
                 ├─ echo 36049 > pr/pr_number
                 ├─ echo fc4... > pr/pr_sha
                 └─ upload-artifact: pr_info
                    └─ Artifact ID: 12345

2. Codebuild completes ──► workflow_run event fires
                            │
                            └─ github.event.workflow_run.id = 19342493369

3. PR Linter triggered ──► download-if-workflow-run job
                            │
                            ├─ Download artifact from run 19342493369
                            │  └─ Finds pr_info artifact (ID: 12345) ✓
                            │
                            ├─ Extract files:
                            │  ├─ pr/pr_number = 36049 ✓
                            │  └─ pr/pr_sha = fc4... ✓
                            │
                            └─ Pass to validate-pr job:
                               └─ PR_NUMBER=36049 ✓
                               └─ PR_SHA=fc4... ✓

4. validate-pr job ──► Validates PR #36049 ✓
```

SUMMARY:
--------
The solution works because it:
  1. Captures PR context when it's available (Codebuild)
  2. Persists it in an artifact (survives context switch)
  3. Restores it when needed (PR Linter)
  4. Bypasses GitHub's pull_requests array limitation

See #36055 for details


## How do I validate 

### Test Environment
Validated on fork repository: pahud#16

### Validation Steps

1. **Applied the fix** to fork's `codebuild-pr-build.yml` (artifact upload steps)
2. **Created test PR** to trigger workflows
3. **Verified Codebuild workflow** uploaded `pr_info` artifact successfully
4. **Verified PR Linter workflow** downloaded artifact and applied label

### Results

✅ **Codebuild PR Build**: Successfully uploaded `pr_info` artifact containing:
- `pr_number`: 16
- `pr_sha`: ebeaa4c...

✅ **PR Linter**: Successfully validated PR and applied label
- Workflow run: https://github.com/pahud/aws-cdk/actions/runs/19380492593
- Label applied: `pr/needs-community-review`
- Status: ✅ Success

### Evidence

**Before fix:**
- PR Linter failed with "Cannot determine PR number"
- No `pr_info` artifact available
- workflow_run.pull_requests array was empty

**After fix:**
- PR Linter successfully downloaded `pr_info` artifact
- Correctly identified PR number and SHA
- Applied appropriate review label

### Note on Fork Testing

For fork testing only, I added a `pull_request_target` fallback to `pr-linter.yml` since `workflow_run` events don't trigger from fork workflows. This fallback is **not needed** for aws/aws-cdk as workflows run in the base repository context where `workflow_run` works correctly.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: pahud

.github/workflows/codebuild-pr-build.yml

@@ -89,3 +89,20 @@ jobs:
           path: |
             ~/.docker-images.tar
           key: docker-cache-${{ runner.os }}
+          
+      - name: Save PR info for PR Linter
+        if: github.event_name == 'pull_request'
+        env:
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+          PR_SHA: ${{ github.event.pull_request.head.sha }}
+        run: |
+          mkdir -p ./pr
+          echo $PR_NUMBER > ./pr/pr_number
+          echo $PR_SHA > ./pr/pr_sha
+
+      - name: Upload PR info artifact
+        if: github.event_name == 'pull_request'
+        uses: actions/upload-artifact@v5
+        with:
+          name: pr_info
+          path: pr/