fix: let NoValidTimestampsError propagate from worker for 422 response

[Blackgaurd]

Fixes integration test `test_weekend_only_range` returning 500 instead of 422.

What changed

• `execute_job()` in `service/worker.py` now re-raises `NoValidTimestampsError` instead of catching it into a `JobResult`
• Updated `test_no_valid_timestamps_raises` to expect the exception to propagate via `pytest.raises`

Why

The orchestrator was wrapping `NoValidTimestampsError` in `RuntimeError`, so `routes.py` never saw the original exception type and returned 500 instead of 422.

🤖 Generated with Claude Code

[Blackgaurd]

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• fix: let NoValidTimestampsError propagate from worker for 422 response #325 👈 (View in Graphite)
• refactor: replace _build_recursive with orchestrator #323 : 1 other dependent PR (#324 )
• feat: add build orchestrator #322
• feat: add build worker that executes a single job #321
• feat: add topological scheduler with Kahn's algorithm #320
• feat: add dependency graph collector with range expansion #319
• refactor: add job descriptor and build plan types #318
• main

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