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Title *

Description

## Parent Epic: #195

## Description
Create testing framework for app developers to test apps before deployment.

## Location
engine/apps/testing/sandbox.py, engine/apps/testing/mock_context.py

## AppSandbox Class

### Constructor
- app_class: App class to test
- Creates isolated test environment

### test_action() Method
```python
def test_action(
    action_name: str,
    config: dict | None = None,
    secrets: dict | None = None,
    **kwargs
) -> dict:
    '''
    Returns:
        {
            'result': <action result>,
            'events': [<emitted events>],
            'logs': [<log entries>],
            'checkpoints': {<checkpoint data>},
            'http_requests': [<made requests>],
        }
    '''
```

### Features
- Mock HTTP responses
- Capture emitted events
- Capture log entries
- Capture checkpoints
- Simulate replays (test idempotency)
- Timeout testing

## MockAppContext Class

### Captured Data
- events: list[dict]
- logs: list[dict]
- checkpoints: dict[str, Any]
- http_requests: list[dict]
- state: dict[str, Any]

### Mock HTTP
```python
sandbox = AppSandbox(MyApp)
sandbox.mock_http(
    url='https://api.example.com/*',
    response={'status': 200, 'json': {'ok': True}}
)
```

### Simulate Replay
```python
sandbox.set_checkpoint('step1', {'result': 'cached'})
result = sandbox.test_action('my_action', ...)
# Verify checkpoint was returned, not re-executed
```

## API Endpoint

POST /api/v1/apps/{app_id}/test
- Test app action in sandbox
- Body: {action_name, config, secrets, kwargs, mock_http}
- Permission: developer
- Returns sandbox result

## Testing
- Test sandbox isolation
- Test mock HTTP
- Test checkpoint replay
- Test event capture
- Test log capture

Due Date (YYYY-MM-DD)

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Edit Issue #205