Fix fire-and-forget event tasks in RealtimeSession

[kratos0718]

Summary

RealtimeSession emitted three error events via a bare asyncio.create_task(self._put_event(...)) whose result was discarded (in _on_guardrail_task_done and _on_tool_call_task_done).

asyncio only keeps a weak reference to a task, so a fire-and-forget task can be garbage-collected before it runs (documented behavior). When that happens here, the error event it was meant to deliver is silently dropped — and these are exactly the paths that report guardrail/tool-call failures to the caller.

The session already uses a keep-a-reference pattern for _guardrail_tasks and _tool_call_tasks (a tracking set + add_done_callback(discard)). This change applies the same pattern to the event-emitting tasks via a small _emit_event_soon helper, and cancels any still-pending event tasks during _cleanup.

Test plan

• Added tests/realtime/test_session_event_tasks.py:
  • _emit_event_soon holds a strong reference while the task is pending, delivers the event to the queue, and releases the reference once done.
  • _cleanup_event_tasks cancels pending event tasks.
• make format, make lint pass (ruff).

Issue number

N/A — found via static analysis of fire-and-forget create_task usage.

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation (n/a — internal behavior)
• I've run make lint and make format
• I've made sure tests pass