[Bug]: Scheduled sessions persist an empty workspace_id

### Steps to Reproduce

1. Install AgentScope with the service dependencies:

```shell
python -m pip install "agentscope[service]==2.0.4"
```

2. Save the following as `repro_schedule_workspace.py`:

```python
import asyncio
from typing import Any

from agentscope.app._manager import SchedulerManager
from agentscope.app.message_bus import InMemoryMessageBus
from agentscope.app.storage import (
ChatModelConfig,
ScheduleData,
ScheduleRecord,
ScheduleSource,
SessionConfig,
SessionRecord,
SessionSource,
)

class CapturingStorage:
def __init__(self) -> None:
self.created_config: SessionConfig | None = None

async def upsert_session(
self,
*,
user_id: str,
agent_id: str,
config: SessionConfig,
state: Any,
source: SessionSource,
source_schedule_id: str | None,
session_id: str | None = None,
) -> SessionRecord:
self.created_config = config
return SessionRecord(
id=session_id or "scheduled-session",
user_id=user_id,
agent_id=agent_id,
config=config,
state=state,
source=source,
source_schedule_id=source_schedule_id,
)

async def main() -> None:
storage = CapturingStorage()
scheduler = SchedulerManager(
storage=storage, # type: ignore[arg-type]
message_bus=InMemoryMessageBus(),
)

schedule = ScheduleRecord(
id="schedule-1",
user_id="user-1",
agent_id="agent-1",
data=ScheduleData(
name="workspace reproduction",
description="Reproduce the scheduled workspace issue.",
cron_expression="* * * * *",
chat_model_config=ChatModelConfig(
type="dummy",
credential_id="dummy",
model="dummy",
parameters={},
),
stateful=False,
source=ScheduleSource.AGENT,
source_session_id="source-session",
),
)

trigger = scheduler._build_trigger(schedule)
await trigger()

assert storage.created_config is not None
print(f"source_session_id: {schedule.data.source_session_id!r}")
print(f"scheduled workspace_id: {storage.created_config.workspace_id!r}")

asyncio.run(main())
```

3. Run the script:

```shell
python repro_schedule_workspace.py
```

4. Actual output:

```text
source_session_id: 'source-session'
scheduled workspace_id: ''
```

5. Expected output:

### Environment