fix: Token endpoint response for invalid_client

Author: AnkeshThakur

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

@@ -97,7 +97,7 @@ async def handle(self, request: Request):
             # Authentication failures should return 401
             return PydanticJSONResponse(
                 content=TokenErrorResponse(
-                    error="unauthorized_client",
+                    error="invalid_client",
                     error_description=e.message,
                 ),
                 status_code=401,