Some authorization servers and WAFs reject httpx's default User-Agent on
the OAuth token endpoint. mcp_servers.<name>.oauth.user_agent now stamps a
custom User-Agent onto the two token-endpoint requests (authorization-code
exchange and refresh) on both provider construction paths. Opt-in,
per-server, token requests only — never MCP traffic or discovery, and no
other headers are configurable. Empty/null/non-string values are ignored.
Completes the second half of #75576 (the CIMD half landed via #89566).
Teknium