Fix missing simulation job polling status

[anth-volk]

Fixes #474

Summary

• Treat job IDs absent from the gateway metadata store as 404s before asking Modal for a call result.
• Map Modal job-not-found and expired-output errors to 404 for jobs that were previously issued by the gateway.
• Preserve 202 for running jobs and redacted 500s for actual worker failures.
• Add gateway unit coverage for unknown jobs, lazy Modal handles without metadata, running jobs, expired output, Modal not-found, and worker errors.

Testing

• uv run pytest tests/gateway/test_endpoints.py tests/gateway/test_auth.py -q
• uv run ruff check src/modal/gateway/endpoints.py tests/gateway/test_endpoints.py fixtures/gateway/test_endpoints.py
• uv run ruff format --check src/modal/gateway/endpoints.py tests/gateway/test_endpoints.py fixtures/gateway/test_endpoints.py