Fix: Implement creative assignment in update_media_buy

[bokelley]

Summary

Fixes the update_media_buy endpoint to properly handle creative assignments. Previously, the endpoint ignored creative_ids in package updates and returned empty affected_packages arrays.

Problem

The update_media_buy implementation was incomplete:

• ❌ Ignored creative_ids in package updates
• ❌ Returned empty affected_packages array
• ❌ Changes didn't persist to database
• ❌ Test agent couldn't verify creative assignments

Solution

Added complete creative assignment logic to _update_media_buy_impl:

1. Validation: Checks that all creative IDs exist before assignment
2. Diff Calculation: Determines added/removed creatives from existing state
3. Persistence: Updates CreativeAssignment table
4. Response: Returns proper affected_packages with PackageUpdateResult

Example Response

{
  "media_buy_id": "buy_123",
  "buyer_ref": "buyer_ref_123",
  "affected_packages": [
    {
      "buyer_package_ref": "pkg_default",
      "changes_applied": {
        "creative_ids": {
          "added": ["creative_1", "creative_2"],
          "removed": [],
          "current": ["creative_1", "creative_2"]
        }
      }
    }
  ]
}

AdCP Compliance

✅ Fully compliant with AdCP v2.2.0 specification

• affected_packages includes buyer_package_ref + changes_applied
• creative_ids shows added/removed/current arrays
• Matches PackageUpdateResult interface from spec

Testing

Unit Tests (4 tests)

• ✅ Verify affected_packages structure
• ✅ Test creative addition and replacement
• ✅ Test serialization

uv run pytest tests/unit/test_update_media_buy_affected_packages.py -v
# ✅ 4 passed

Integration Tests (3 tests - requires PostgreSQL)

• ✅ Test database persistence
• ✅ Test creative validation (reject missing IDs)
• ✅ Test creative replacement workflow

./run_all_tests.sh ci --test-path tests/integration/test_update_media_buy_creative_assignment.py

Files Changed

• src/core/main.py: +80 lines (creative assignment logic)
• tests/unit/test_update_media_buy_affected_packages.py: 4 unit tests
• tests/integration/test_update_media_buy_creative_assignment.py: 3 integration tests
• docs/fixes/2025-10-23-update-media-buy-creative-assignment.md: Complete fix documentation
• docs/testing/postmortems/2025-10-23-test-agent-update-media-buy-stub.md: Test agent issue documentation
• CLAUDE.md: Updated known test agent issues section

Impact

• ✅ MCP update_media_buy tool now supports creative assignments
• ✅ A2A update_media_buy endpoint now supports creative assignments
• ✅ Test agent will work correctly (uses same codebase)
• ✅ Enables end-to-end creative assignment testing

Test Plan

• Unit tests verify schema compliance
• Integration tests verify database persistence
• Pre-commit hooks pass
• Pre-push tests pass (838 unit + 174 integration)
• Manual testing with test agent (after deployment)
• E2E creative assignment workflow verification

Documentation

• Fix documentation
• Test agent issue postmortem

🤖 Generated with Claude Code