test: expand Python backend coverage to ~52%

[gf5901]

Summary

• Add targeted tests for under-covered modules: PR verification/body generation, health router edge cases, watcher loop resilience, worktree/agent context helpers, PM agent normalization, and healer stale-worktree cleanup
• Extract shared test helpers into tests/support.py (mock_process, git_cmd_side_effect, attach_pr_mocks) to reduce boilerplate in PR pipeline tests
• Move the FastAPI client fixture from test_api.py into conftest.py so all API test files can reuse it
• Fix flaky watcher loop test by using asyncio.Event signaling instead of wall-clock sleep
• Remove duplicate empty-project-id DynamoDB test

Coverage: ~48% → ~52% overall; notable jumps in pr.py (39→76%), agent.py (55→63%), watcher.py (59→91%), pm_agent.py (0→29%), healer.py (70→88%).

Test plan

• Full pytest tests/ suite passes locally (381 passed)
• ruff check clean on all new/modified files
• No production code changes — test-only diff

Made with Cursor