Handler-generated HeaderMismatch responses use HTTP 200 instead of 400 #1225

Describe the bug

When a modern Streamable HTTP request reaches a ServerHandler and the handler returns ErrorData::header_mismatch(...), RMCP serializes the expected JSON-RPC HeaderMismatch error (-32020) but sends it with HTTP 200.

The modern response path derives the HTTP status through jsonrpc_http_status, which maps several protocol errors to HTTP 400 but currently falls through to HTTP 200 for ErrorCode::HEADER_MISMATCH. Transport-generated header validation errors do not show the bug because header_mismatch_jsonrpc_response constructs an HTTP 400 response directly.

To Reproduce

1. Implement a ServerHandler request method that returns Err(ErrorData::header_mismatch("header does not match request parameter", None)).
2. Send a valid modern Streamable HTTP JSON-RPC request using protocol version 2026-07-28, so the request reaches the handler.
3. Observe an HTTP 200 response whose JSON-RPC body contains error code -32020.

Expected behavior

Handler-generated HeaderMismatch errors on the modern per-request path should use HTTP 400 while preserving JSON-RPC error code -32020, as required by the Streamable HTTP server-validation rules:

https://modelcontextprotocol.io/specification/2026-07-28/basic/transports/streamable-http#server-validation

Logs

actual:   HTTP 200, JSON-RPC error -32020
expected: HTTP 400, JSON-RPC error -32020

Additional context

The missing mapping is in:

https://github.com/modelcontextprotocol/rust-sdk/blob/main/crates/rmcp/src/transport/streamable_http_server/tower.rs

Related work such as #1083 covers mismatches detected by the transport before handler dispatch. This issue is specifically about HeaderMismatch returned by application handlers after dispatch.