Add error handling for A2A conversation messages endpoint

[devin-ai-integration]

Summary

Adds explicit error handling to the GET /public/apps/<app_id>/a2a/conversations/<conversation_id>/messages route.

Before: An invalid conversation_id (non-UUID string) caused an unhandled ValueError in the service layer, resulting in a generic 500 error. An invalid app_id raised NotFoundException which was caught by the global error handler but returned HTTP 200 with a business error code — inconsistent with the A2A pattern of returning proper HTTP status codes (e.g. 503 for service unavailable).

Changes:

• Service layer (public_agent_a2a_service.py): Wraps UUID(conversation_id) in a try/except to convert ValueError/AttributeError into a ValidateErrorException with a descriptive message.
• Handler layer (public_app_handler.py): Catches NotFoundException → 404, ValidateErrorException → 400, and generic Exception → 500, returning JSON error responses consistent with the existing A2A pattern (e.g. the 503 already returned when the service is unavailable).

Review & Testing Checklist for Human

• Verify the existing test test_public_app_a2a_conversation_messages_should_delegate_to_service still passes in CI — local tests could not be run due to missing PostgreSQL/Redis infrastructure.
• Decide if sibling A2A endpoints (get_public_app_a2a_card, send_public_app_a2a_message, get_latest_public_app_a2a_conversation) should get the same error handling — currently they let NotFoundException bubble to the global handler which returns HTTP 200 with a business error code, creating an inconsistency.
• Review whether the bare except Exception catch-all in the handler is desired — it prevents unexpected errors from reaching the global handler. This is consistent with returning A2A-style JSON errors, but could mask programming bugs during development.

Suggested manual test plan: call the endpoint with (1) a valid app_id + valid conversation_id, (2) a non-UUID app_id, (3) a valid app_id + non-UUID conversation_id, (4) a valid UUID that doesn't correspond to any app, and verify you get 200/404/400/404 respectively.

Notes

• The happy path (valid parameters, successful query) is unchanged and still returns success_json(messages).

Link to Devin session: https://app.devin.ai/sessions/c588027acf0a490c8b3467ef8f8fe02b
Requested by: @Haohao-end

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring