fix: Token endpoint response for invalid_client (#1481)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

src/mcp/server/auth/handlers/token.py

@@ -97,7 +97,7 @@ class TokenHandler:
             # Authentication failures should return 401
             return PydanticJSONResponse(
                 content=TokenErrorResponse(
-                    error="unauthorized_client",
+                    error="invalid_client",
                     error_description=e.message,
                 ),
                 status_code=401,