Emit history updates for realtime transcript deltas

[mindbomber]

Fixes #2940

Summary

• Emit RealtimeHistoryUpdated after transcript_delta mutates realtime history.
• Keep raw model event emission unchanged for lower-level streaming consumers.
• Add regression coverage that verifies transcript deltas update assistant audio transcript history and emit the transformed history event.

Verification

• PYTHONPATH=src python -m pytest tests/realtime/test_session.py -q -> 73 passed
• python -m ruff check src/agents/realtime/session.py tests/realtime/test_session.py -> passed
• git diff --check -> passed
• AANA code-change guardrail -> pass / accept, AIx 1.0, no violations

[seratch]

duplicate with #3086