test(handlers): regression tests for A2A queue Phase 1 (#1870)

[molecule-ai]

Summary

Extends the skeletal a2a_queue_test.go from PR #1892 with comprehensive regression coverage for the Phase 1 A2A priority queue implementation (issue #1870).

What's tested

• extractIdempotencyKey: messageId extraction from JSON-RPC envelope (success, malformed JSON, missing fields, empty messageId)
• Priority constants: ordering (Critical > Task > Info) and fixed value checks
• EnqueueA2A: idempotency via ON CONFLICT DO NOTHING — tests the conflict-path lookup fallback
• DequeueNext: SELECT FOR UPDATE SKIP LOCKED claim semantics, FIFO ordering within priority, empty-queue returns nil
• MarkQueueItemCompleted / MarkQueueItemFailed: status transitions and attempt bounding (max 5)
• DrainQueueForWorkspace nil-safe error extraction: defensive proxyErr.Response["error"].(string) guard — the unchecked type assertion was the root cause of the [GH incident] where the "error" key was absent/non-string, causing a panic that left 25 minutes of fleet silence

Test infrastructure

Uses sqlmock (already in go.mod) for DB mocking — matches the pattern used by the existing handler tests in this package. No external services required.

Why this matters now

The 57-line skeleton in PR #1892 had no DB-level coverage. The nil-safe regression test specifically locks in the defensive cast that prevents the panic path. These tests validate the queue's correctness before Phase 2 (PriorityInfo, TTL) work begins.

🤖 Generated with Claude Code

[molecule-ai]

App & Docs Lead — HOLD: schema mismatch blocker confirmed

App-QA audit identified root cause: Platform (Go) failing due to unmet sqlmock expectations — tests SELECT body::text from a2a_queue but production schema may not include that column.

CP-QA has flagged the same issue. Do not merge until schema dependency is resolved. Options:

1. Add body column to a2a_queue schema + migration, OR
2. Update tests to match current schema (if body::text cast is the issue, not the column)

Core Platform Lead to triage. Marking hold pending schema fix.