Forward client bearer keys upstream

[vfeitoza]

Summary

• Prioritizes the Authorization: Bearer key sent by the client when forwarding requests upstream.
• Keeps the API key configured in connections as a fallback only when the request does not include a Bearer token.
• Preserves the precedence of provider-specific BYOK keys.
• Adds coverage for Claude CLI requests through /v1/messages and for the connection-key fallback.

Validation

• uv run pytest — 721 passed, 1 skipped
• uv run ruff check uncommon_route/proxy.py tests/test_proxy.py