chore(tests): remove unused import of MAX_CONTENT_LENGTH in test_serialization.py

Author: sokoliva

tests/server/apps/jsonrpc/test_serialization.py

@@ -6,7 +6,6 @@
 from pydantic import ValidationError
 from starlette.testclient import TestClient
 
-from a2a.server.apps.jsonrpc.jsonrpc_app import MAX_CONTENT_LENGTH
 from a2a.server.apps import A2AFastAPIApplication, A2AStarletteApplication
 from a2a.types import (
     APIKeySecurityScheme,