Apply start_retriable to agent background tasks

## Overview

Apply the `start_retriable` pattern to agent-related background tasks as part of the Redis-based background task migration.

## Acceptance Criteria

- All agent background tasks use `start_retriable` for retry logic
- Consistent error handling and retry behavior across agent tasks
- Tests verify retry mechanism works correctly

## Technical Notes

- Review existing agent background task implementations
- Replace current retry mechanisms with `start_retriable`
- Ensure proper error handling and logging

## Implementation Structure

Follow the same structure as `src/ai/backend/storage/bgtask`:

```
src/ai/backend/agent/bgtask/
├── __init__.py
├── registry.py # BgtaskHandlerRegistryCreator - register all handlers
├── tags.py # Task tag constants (if needed)
└── tasks/ # Individual task handler modules
├── __init__.py
└── {task_name}.py # Each handler inherits BaseBackgroundTaskHandler
```

**Task Handler Pattern:**

- Define `Args` class inheriting `BaseBackgroundTaskArgs`
- Define `Handler` class inheriting `BaseBackgroundTaskHandler[ArgsType]`
- Implement `name()`, `args_type()`, `execute()` methods
- Register handlers in `registry.py`

JIRA Issue: BA-2689