Add a turn/response group identifier to streaming events

[ferponse]

Problem

When using runner.run_async() with StreamingMode.SSE, there is no way to distinguish which partial streaming chunks belong to which LLM response turn within a single invocation.

In a typical flow where the LLM responds with text, calls a tool, and then responds with more text, the events look like this:

Event(partial=True,  text="Looking up...")      # LLM call 1
Event(partial=True,  text="Looking up info...")  # LLM call 1
Event(partial=False, function_call=...)          # LLM call 1 (final)
Event(function_response=...)                     # Tool result
Event(partial=True,  text="Here are the...")     # LLM call 2
Event(partial=True,  text="Here are the results...")  # LLM call 2
Event(partial=False, text="Here are the results...")  # LLM call 2 (final)

All of these events share the same invocation_id, and the id field changes on every yield. There is no field to programmatically determine that the first two partial chunks belong to LLM call 1 and the last two belong to LLM call 2.

Current workaround

The only way to detect turn boundaries is by observing transition patterns: a partial=False event followed by function call/response events implicitly signals the end of one text group. This is fragile and requires the consumer to maintain state tracking.

Proposed solution

Add a turn_id field to Event that:

• Is a 1-based integer counter incremented with each LLM call inside run_async
• Remains the same across all partial chunks and the final aggregated event of that LLM call
• Changes when a new LLM call starts (e.g., after tool execution)

Use case

We build a chat UI that streams partial text to the user via WebSocket. When the agent produces text, calls a tool, and then produces more text, we need to render them as separate message bubbles. Without a turn identifier, we cannot cleanly separate the two text blocks on the client side without implementing brittle heuristics based on event type transitions.