Unix-socket HTTP client matches response media types by case-sensitive prefix #1443

Follow-up requested in the review of #1390 by @jokemanfire.

The Unix-socket backend still uses case-sensitive prefix checks for response Content-Type in crates/rmcp/src/transport/common/unix_socket.rs, in POST JSON/SSE dispatch and GET admission. Current main has the comparisons at lines 323, 328 and 511–512.

By code inspection, this rejects valid case variants such as Application/JSON and Text/Event-Stream, and admits different media types such as application/jsonx and text/event-streaming. Parameters such as application/json; charset=utf-8 should continue to work.

Suggested scope is to compare complete media-type essences case-insensitively in the Unix-socket client, preserving parameter tolerance, with Unix-socket transport regressions. The matcher introduced by #1390 could move to common/http_header.rs for reuse after that PR lands.

This report is based on inspection of current main and the maintainer review. I have not run a Unix-socket wire reproduction. The reqwest wire regressions in #1390 establish the corresponding behavior only for that backend.

Server request admission was handled separately by merged #1382.