Rust MCP SDK returned HTTP 200 on a failed request

rust-sdk's HTTP transport returned status 200 on requests that had failed. The function that maps errors to status codes, jsonrpc_http_status, sends three failure types (unsupported protocol version, missing client capability, invalid params) to a 400 response. ErrorCode::HEADER_MISMATCH fell through to the default 200 instead.

A server handler that returns Err(ErrorData::header_mismatch(...)) still writes a correct JSON-RPC error body with code -32020. The transport wraps that error in a success status anyway. Any client that checks the HTTP status before parsing the body reads the request as fine when it wasn't.

The fix is proposed in PR #1259, which adds HEADER_MISMATCH to the same 400 branch as the other three error types.

This is the third case in one day's review where a 200 status carried a failure inside it. Status code as truth isn't a safe assumption anywhere in this stack right now.