fix: emit RealtimeHistoryUpdated on transcript_delta

[jordanchendev]

Summary

RealtimeSession updated _history with the accumulated transcript on each transcript_delta event but never emitted RealtimeHistoryUpdated. Every other branch that mutates history (item_updated, item_deleted, transcription_completed) does emit the event, so UI subscribers following the documented history_added / history_updated pattern saw no live transcript changes during streaming.

This fix emits RealtimeHistoryUpdated immediately after the history update in the transcript_delta branch, consistent with all other history-mutating paths.

Test plan

• Added regression test test_transcript_delta_emits_history_updated that verifies both a RealtimeRawModelEvent and a RealtimeHistoryUpdated are emitted for each transcript_delta event.
• Updated test_ignored_events_only_generate_raw_events to reflect the corrected behavior (4 events instead of 3: transcript_delta now produces a raw event + a history update).
• uv run pytest tests/realtime/ -q → 256 passed
• make format && make lint → all checks passed

Issue number

Closes #2940

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass