feature: resume scheduled task automatically

[vcfgv]

📝 Pull Request Template

1. Related Issue

Closes # (issue number)

2. Type of Change (select one)

New Feature

3. Description

• Add stop_reason and stop_reason_detail to StrategyStatusContent and emit it on validation/runtime errors.
• Add a TaskStore abstraction with InMemoryTaskStore and SQLiteTaskStore and refactor TaskManager/TaskService to use it (task_store.py).
• Auto-resume running recurring tasks on server startup and schedule them via TaskExecutor.execute_task (agent_stream.py).
• On task failure, update the persisted scheduled_task_controller conversation item: set payload task_status and add metadata.error_reason for frontend visibility.

4. Testing

• I have tested this locally.
• I have updated or added relevant tests.

5. Checklist

• I have read the Code of Conduct
• I have followed the Contributing Guidelines
• My changes follow the project's coding style