mcp: ignore unknown fields in the client registration request (#5643)

## Summary

Some clients may send RFC7591 Client Registration Request with extra
fields that are not part of the spec, and we used too restrictive
decoder for that. This PR ignores the unknown fields.

## Related issues

<!-- For example...
- #159
-->

## User Explanation

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change doesn't create any user-facing changes, you can leave
this blank. If filled out, add the `docs` label -->

## Checklist

- [ ] reference any related issues
- [ ] updated unit tests
- [ ] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [ ] ready for review

internal/mcp/handler_register_client.go

@@ -35,7 +35,7 @@ func (srv *Handler) RegisterClient(w http.ResponseWriter, r *http.Request) {
 	}
 
 	v := new(rfc7591v1.ClientMetadata)
-	err = protojson.Unmarshal(data, v)
+	err = protojson.UnmarshalOptions{DiscardUnknown: true}.Unmarshal(data, v)
 	if err != nil {
 		log.Ctx(ctx).Error().Err(err).Msg("failed to unmarshal request body")
 		http.Error(w, "failed to unmarshal request body", http.StatusBadRequest)