fix(client): parse CONNECT responses and preserve early data

[hunterinvariants]

The HTTP CONNECT helper previously recognized responses using byte-prefix checks and required the header terminator to occur at the end of a read buffer. This could accept malformed status lines and mishandle data received immediately after a successful CONNECT response.

This change:

• parses the proxy response with httparse;
• returns a TunnelConnection<C::Response> that replays bytes read beyond the response headers;
• delegates the original connection metadata through the buffered connection;
• adds deterministic regressions for malformed status codes and early tunneled data.

Tunnel<C> now uses TunnelConnection<C::Response> as its Service::Response, since retaining bytes already read from the destination requires a buffered return type.

Verified with the complete all-features test suite.

Fixes hyperium/hyper#4095