feat(bridge): add anthropic-compatible gateway surface

[typelicious]

Summary

• add a minimal Anthropic bridge scaffold with canonical request and response models
• expose Anthropic messages and count_tokens through the existing Gate routing path
• add an optional Claude Code routing hook plus config, docs, and a local smoke example

Testing

• env PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_config.py tests/test_anthropic_api.py tests/test_anthropic_bridge.py tests/test_request_hooks.py
• rtk ruff check faigate/config.py faigate/main.py tests/test_config.py tests/test_anthropic_api.py
• bash -n docs/examples/anthropic-bridge-smoke.sh

In general, the fix is to avoid returning raw exception messages to the client. Instead, log the detailed exception server-side and send the client a generic, non-sensitive message (or, at most, a high-level description that doesn’t include stack traces, paths, or provider internals).

Specifically here, two except AnthropicBridgeError as exc: blocks currently call _anthropic_error_response(str(exc), ...). We should change these to:

• Log the exception (including stack trace if desired) using the existing logger.
• Return a generic error message (for example “Invalid Anthropic messages request” / “Invalid Anthropic count_tokens request” or “Anthropic bridge request failed”) that does not depend on exc.

Concretely, in faigate/main.py:

1. Around lines 3000–3005 (Anthropic messages bridge route), replace the call that returns _anthropic_error_response(str(exc), ...) with:

   • A logger.warning (or logger.error) including the exception object.
   • A call to _anthropic_error_response with a fixed message such as "Invalid Anthropic messages request" and the same error_type and status_code as before.

2. Around lines 3085–3089 (Anthropic count_tokens route), do the same: log exc, and return _anthropic_error_response with a fixed, non-tainted message such as "Invalid Anthropic count_tokens request".

No new imports are needed; logger already exists. This preserves HTTP semantics and error types while preventing exception-derived content from reaching clients.