[Server] Mcp-Param header is accepted when the body omits its argument #533

StandardHeaderValidator::checkParam() returns early when the argument mirrored via x-mcp-header is absent from the body, without looking at the header. So a tools/call with Mcp-Param-Region: us-west1 but no region argument (or no arguments at all) passes validation, even though header and body disagree.

SEP-2243 has the client omit the header when the argument is absent, and the server validate recognized headers against the body => this should be rejected with -32020 like any other mismatch. The Python SDK does that, the TypeScript SDK currently ignores it like we do - I'd align with Python here. Unknown Mcp-Param-* headers stay ignored.

Thanks @GEONWOOHAN for the report!