fix(streaming): prefer event.task_id when building StreamChunk.task_id

[llaoj]

Summary

This PR makes StreamChunk.task_id use event.task_id as the primary source, with a backward-compatible fallback to current_task_info["id"].

• Before:
  • StreamChunk.task_id could be empty when current_task_info["id"] was empty.
• After:
  • StreamChunk.task_id = event.task_id or current_task_info["id"] or "".

Why this matters

LLMStreamChunkEvent.task_id is populated from from_task in event normalization, while current_task_info["id"] may remain empty in some streaming contexts.

So even when task identity exists in the event, downstream consumers can still receive chunks with empty chunk.task_id if StreamChunk.task_id is built only from current_task_info.

Real downstream example (SSE filtering)

In a downstream FastAPI SSE endpoint, we collect crew task IDs and filter stream chunks by chunk.task_id:

crew = CrewAgent().crew()
task_id_set = {str(task.id) for task in crew.tasks}

async for chunk in streaming:
    content = getattr(chunk, "content", None)
    task_id = getattr(chunk, "task_id", None)
    if content and task_id in task_id_set:
        await queue.put(f"data: {json.dumps(content, ensure_ascii=False)}\n\n")

If chunk.task_id is empty, valid chunks can be dropped by filtering logic.

Related concurrency concern (important, not fully solved by this PR)

While each request/run typically has its own queue, stream handlers are registered on the singleton crewai_event_bus.
That means with concurrent streaming runs:

• Request A registers handler_A -> writes to queue_A
• Request B registers handler_B -> writes to queue_B
• A single LLMStreamChunkEvent can be dispatched to all active handlers

So this is not a "shared queue" problem; it is an event fan-out to multiple handlers problem on a global event bus, which can cause cross-run chunk contamination if handler-level scoping/filtering is missing.

This PR addresses task_id correctness in StreamChunk.
A follow-up improvement could add stronger run/stream scoping for handler dispatch.

Compatibility

Backward compatible:
If event.task_id is missing, existing fallback behavior (current_task_info["id"]) is preserved.

Test plan

• Verify stream chunks now carry non-empty task_id when event contains task context.
• Verify fallback still works when event.task_id is unavailable.
• Validate downstream filtering by task IDs no longer drops valid chunks due to empty task_id.