fix(serverless): allow GET status requests

[tgolob]

Closes #369

Summary

• allow the local FastAPI /status/{job_id} endpoint to accept GET requests, matching the public RunPod API
• keep POST compatibility and add a route-registration regression test

Validation

• pytest tests/test_serverless/test_modules/test_fastapi.py -q --no-cov (9 passed)
• python -m compileall -q runpod tests/test_serverless/test_modules/test_fastapi.py
• git diff --check
• Ruff was not available in the local test environment