Python: Add a typed schema for AgentExecutor checkpoint state

## Summary

Add a public typed schema for the checkpoint state produced and restored by `AgentExecutor`.

## Current behavior

The generic executor checkpoint hooks use `dict[str, Any]`:

```python
async def on_checkpoint_save(self) -> dict[str, Any]: ...
async def on_checkpoint_restore(self, state: dict[str, Any]) -> None: ...
```

`AgentExecutor.on_checkpoint_save()` currently returns a mapping containing:

- `cache`
- `full_conversation`
- `agent_session`
- `pending_agent_requests`
- `pending_responses_to_agent`

The payload is stored under `WorkflowCheckpoint.state["_executor_state"][executor_id]`. The shape is not currently represented by a public `TypedDict` or equivalent model.

## Request

Please introduce and document a typed checkpoint-state contract for `AgentExecutor`, including:

1. A public `TypedDict` or equivalent type for the serialized state.
2. Typed handling for the serialized `AgentSession` payload.
3. Clear compatibility/versioning guidance for adding or removing fields.
4. Validation and useful errors for malformed checkpoint data during restore.
5. Tests covering serialization, restoration, missing optional fields, and backward compatibility.
6. Guidance for custom executors that implement their own checkpoint state.

## Rationale

Applications that need to inspect, migrate, clean up, or associate executor checkpoint data with application sessions currently have to rely on an undocumented `dict[str, Any]` shape. A typed contract would improve static analysis and make checkpoint persistence safer to evolve without confusing internal state with a stable public API.