fix(realtime): emit history_updated for transcript deltas

[LocNguyenSGU]

Summary

RealtimeSession.on_event() already updates local history when it receives a transcript_delta, but it did not emit a matching high-level history event.

This patch emits RealtimeHistoryUpdated after the transcript-delta history mutation so consumers that follow history_added / history_updated stay in sync with live transcript updates.

Reproduction

A single RealtimeModelTranscriptDeltaEvent produced:

• a mutated _history entry containing the partial assistant transcript
• only a raw model event in the queue
• no RealtimeHistoryUpdated

Changes

• emit RealtimeHistoryUpdated in the transcript_delta branch after updating _history
• add a focused regression test for the high-level event contract
• narrow the existing ignored-events test so it only covers truly ignored events

Verification

uv run pytest tests/realtime/test_session.py -k "transcript_delta_updates_history_and_emits_history_updated or ignored_events_only_generate_raw_events or transcript_delta_triggers_guardrail_at_threshold or transcript_delta_multiple_thresholds_same_item or transcript_delta_different_items_tracked_separately" -q

Result:

• 5 passed

Risks

Low. The change only adds the missing high-level event emission for an already-applied history mutation.

Closes #2940

[afurm]

Line 318: history=self._history passes a direct reference to the session's internal _history list rather than a copy. If any subscriber mutates this list, it silently corrupts RealtimeSession._history without the session knowing.

Consider: history=list(self._history) to prevent external mutation from affecting internal state.

[LocNguyenSGU]

Fixed in 7f3eff8. RealtimeHistoryUpdated now receives list(self._history), so consumers can no longer mutate the session's internal history list through the event payload.

I also made the snapshot behavior consistent across all full-history RealtimeHistoryUpdated emissions and added a regression test for the transcript_delta path.

[LocNguyenSGU]

Fixed the lint formatting issue in tests/realtime/test_session.py.

Verified locally:

• python3 -m ruff format --check
• python3 -m ruff check

Commit: 7d2c6a83