Verify and Fix Message Queue Event Request ID Propagation

## Motivation

When events are dispatched through the message queue system, the request ID context may not be properly propagated to event handlers. This causes request ID to show as "unknown" in event processing logs.

- `MessageMetadata.apply_context()` may not be called in all event consumption paths
- Some event handlers may not preserve request ID when spawning sub-tasks

## Required Features

- Audit all event dispatcher and consumer code paths for proper context application
- Ensure `MessageMetadata` is properly serialized with current request ID when producing events
- Ensure `metadata.apply_context()` is called when consuming events
- Fix any identified gaps in context propagation

## Impact

- `src/ai/backend/common/message_queue/types.py`
- `src/ai/backend/common/events/dispatcher.py`
- Event consumers across manager, agent, and other components

## Testing Scenarios

- Verify request ID is preserved through event publish/subscribe cycle
- Test with EventDispatcher producing events with request context
- Verify logs in event handlers show correct request ID

JIRA Issue: BA-3722