continue_conversation=True starts a new session instead of resuming the previous one

In the Python Agent SDK, `ClaudeAgentOptions(continue_conversation=True)` does not resume the previous session after restarting the script.

A new `session_id` is created on every run, even when:
- the same script is used
- the same `cwd` is used
- the previous run fully completes and reaches `ResultMessage`

According to the docs, this option should continue the most recent session in the directory.

## Minimal repro

```python
import asyncio
from pathlib import Path
from claude_agent_sdk import query, ClaudeAgentOptions, ResultMessage

PROJECT_DIR = Path(__file__).resolve().parent

async def main():
options = ClaudeAgentOptions(
cwd=str(PROJECT_DIR),
continue_conversation=True,
)

result = None
async for message in query(
prompt="Do you remember what we talked about last time?",
options=options,
):
if isinstance(message, ResultMessage):
result = message

print("session_id:", result.session_id)

asyncio.run(main())
```
Steps
Run the script once
Note the printed session_id
Run it again
The second run gets a different session_id
Expected
The second run should resume the most recent session in the same directory.

Actual
A new session is created each time.