feat(bridge): improve anthropic error and header mapping

[typelicious]

Summary

• improve Anthropic-compatible error typing from upstream status codes
• preserve bridge source and model-resolution details in response headers
• return the final upstream status instead of collapsing all failures into a generic 502

Testing

• env PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_anthropic_api.py tests/test_anthropic_bridge.py tests/test_config.py tests/test_request_hooks.py
• rtk ruff check faigate/main.py tests/test_anthropic_api.py
• rtk ruff format --check faigate/main.py tests/test_anthropic_api.py