CVE-2026-64684: RMCP: Custom HTTP headers leak to cross-origin redirect targets

The rmcp crate’s StreamableHttpClientTransport forwards caller-supplied custom HTTP headers (such as X-API-Key, X-Auth-Token, Api-Key) to cross-origin redirect targets. The default_http_client() function builds a reqwest::Client without a redirect policy override, so the default limited(10) policy follows 307/308 redirects and forwards all per-request headers except Authorization, Cookie, and Proxy-Authorization. Custom auth headers injected via StreamableHttpClientTransportConfig.custom_headers are not classified as sensitive and are therefore forwarded verbatim to any redirect target — including an attacker-controlled server.