fix(realtime): #2940 emit history_added / history_updated on transcript_delta

[abhicris]

Closes #2940.

Bug

RealtimeSession.on_event updates self._history on transcript_delta but only forwards the raw model event to the queue. UIs that follow the documented history_added / history_updated contract therefore never see live partial-transcript changes — even though the session history has already changed.

The repro in the issue shows that after a single RealtimeModelTranscriptDeltaEvent, the queue carries the raw event but no RealtimeHistoryAdded / RealtimeHistoryUpdated.

Fix

Mirror the pattern already used for input_audio_transcription_completed (lines 283-294 of the same file): capture the pre-update history length, apply _get_new_history, and emit:

• RealtimeHistoryAdded if a brand-new item was appended (first transcript delta for an item), or
• RealtimeHistoryUpdated otherwise (subsequent partial-transcript update).

         elif event.type == "transcript_delta":
             item_id = event.item_id
             if item_id not in self._item_transcripts:
                 self._item_transcripts[item_id] = ""
                 self._item_guardrail_run_counts[item_id] = 0

             self._item_transcripts[item_id] += event.delta
+            prev_len = len(self._history)
             self._history = self._get_new_history(
                 self._history,
                 AssistantMessageItem(
                     item_id=item_id,
                     content=[AssistantAudio(transcript=self._item_transcripts[item_id])],
                 ),
             )
+            if len(self._history) > prev_len and len(self._history) > 0:
+                await self._put_event(
+                    RealtimeHistoryAdded(info=self._event_info, item=self._history[-1])
+                )
+            else:
+                await self._put_event(
+                    RealtimeHistoryUpdated(info=self._event_info, history=self._history)
+                )

Both events are already imported (lines 36-37) and used elsewhere in the file. No new state, no public-API change.

+13 / 0. Guardrail-debounce logic below this block is unaffected.

---

Contributed by kcolbchain.

[github-actions]

This PR is stale because it has been open for 10 days with no activity.