fix: Remove fake media_buy_id from pending/async responses

[bokelley]

Summary

Fixed mock adapter to not return fake media_buy_id values for pending/async operations. When a media buy doesn't exist yet, we should return media_buy_id=None, not confusing fake IDs like "pending_xyz".

Problem

The mock adapter was returning fake IDs for async/pending operations:

• "pending_question_{id}" for question-asking scenarios
• "pending_{step_id}" for async mode

This is wrong because:

1. Media buy doesn't exist yet - it's being processed
2. Fake IDs confuse clients - they look queryable but aren't
3. Violates A2A protocol - pending operations should have minimal domain data

Solution

Per AdCP protocol envelope design:

• Domain response returns media_buy_id=None (doesn't exist yet)
• Domain response returns packages=[] (not created yet)
• Protocol envelope provides status="working" or "input-required"
• Protocol envelope provides task_id (workflow_step_id) for tracking

Clients use task_id to poll or wait for webhook, NOT a fake media_buy_id.

Changes

• Mock adapter question-asking: Returns media_buy_id=None, packages=[]
• Mock adapter async mode: Returns media_buy_id=None, packages=[]

Example Responses

Before (wrong):

{
  "status": "working",
  "task_id": "workflow_abc",
  "payload": {
    "media_buy_id": "pending_workflow_abc",  // Fake ID!
    "packages": []
  }
}

After (correct):

{
  "status": "working",
  "task_id": "workflow_abc",
  "payload": {
    "media_buy_id": null,  // No media buy yet
    "packages": []
  }
}

Testing

• Pre-commit hooks pass
• Only mock adapter changes (no schema files)
• Clean rebase on latest main

Notes

This PR replaces #655 which accidentally included schema file changes.

🤖 Generated with Claude Code