- probe_api_models: add api_mode param; use x-api-key + anthropic-version
headers for anthropic_messages mode (Anthropic's native Models API auth)
- probe_api_models: add User-Agent header to avoid Cloudflare 403 blocks
on third-party OpenAI-compatible endpoints
- validate_requested_model: pass api_mode through from switch_model
- validate_requested_model: for anthropic_messages mode, attempt probe with
correct auth; if probe fails (many proxies don't implement /v1/models),
accept the model with an informational warning instead of rejecting
- fetch_api_models: propagate api_mode to probe_api_models