`jsonrpc_http_status` maps several protocol errors (unsupported protocol
version, missing required client capability, invalid params) to HTTP 400
on the modern per-request Streamable HTTP path, but fell through to the
default HTTP 200 for `ErrorCode::HEADER_MISMATCH`. When a `ServerHandler`
returns `Err(ErrorData::header_mismatch(...))`, RMCP serialized the
correct JSON-RPC -32020 error but sent it with HTTP 200.
The legacy/transport-generated header validation path (SEP-2243
Mcp-Method/Mcp-Name/Mcp-Param-* mismatches) was unaffected, since
`header_mismatch_jsonrpc_response` constructs an HTTP 400 response
directly rather than going through `jsonrpc_http_status`. Only
handler-generated HeaderMismatch errors on the modern path hit the bug.
Add `ErrorCode::HEADER_MISMATCH` to the existing `BAD_REQUEST` match arm,
matching the Streamable HTTP server-validation rules in the 2026-07-28
spec. Added unit tests covering the mapper directly (header-mismatch to
400, method-not-found to 404, an unmapped code defaulting to 200).
Closes#1225
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
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