fix: return header mismatch for missing protocol header

[mrcs64]

Return HeaderMismatch (-32020) when request _meta declares a protocol version but the required MCP-Protocol-Version header is absent. Add a stateless HTTP regression test covering the complete JSON-RPC error response.

Motivation and Context

The 2026-07-28 per-request protocol mirrors request metadata into HTTP headers. Required headers that are missing, malformed, or disagree with the request body are header-validation failures.

The missing MCP-Protocol-Version branch previously returned Invalid Request (-32600), unlike the adjacent header/body mismatch branch, which correctly returns HeaderMismatch (-32020).

This change is intentionally limited to that modern per-request validation branch:

• a missing required field in server/discover remains Invalid Params (-32602)
• legacy initialize header/body validation remains Invalid Request (-32600)

References:

• MCP 2026-07-28 transport metadata
• SEP-2243 header validation

How Has This Been Tested?

Added a stateless HTTP regression test that verifies:

• HTTP 400
• JSON-RPC version and request ID
• error code -32020
• a branch-specific missing-header diagnostic

Ran:

cargo test -p rmcp \
  --test test_streamable_http_protocol_version \
  --test test_server_discover_http \
  --test test_streamable_http_standard_headers \
  --features 'server client transport-streamable-http-server reqwest'

Result: 28 passed, 0 failed.

cargo fmt --all -- --check also passes with the installed stable formatter. It reports the repository's existing warnings for nightly-only rustfmt settings.

Breaking Changes

None. This corrects the error classification for an already-invalid request.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The regression request includes the required Mcp-Method header so the only missing modern routing header is MCP-Protocol-Version. The assertion matches a stable message fragment rather than the full diagnostic text.

[DaleSeo]

Thanks for the fix, @mrcs64!