fix(checkpoint): serialize type[BaseModel] fields as JSON schema

Subclass redeclarations of args_schema/response_format dropped the
parent's Annotated PlainSerializer, causing PydanticSerializationError
on model_dump(mode='json'). Replace with @field_serializer decorators
backed by a shared serialize_model_class helper:

- BaseTool: covers RecallMemoryTool, RememberTool, AskQuestionTool,
  DelegateWorkTool, AddImageTool, ReadFileTool
- BaseLLM (check_fields=False): covers LLM, Anthropic, OpenAI, Gemini,
  Bedrock
- LiteAgent.response_format
- A2AConfig / A2AClientConfig response_model