Two issues were causing TTSSpeakFrame(append_to_context=True) greetings to
silently lose their trailing words and never fire on_assistant_turn_stopped:
- LLMAssistantPushAggregationFrame was emitted without a PTS, so the
transport routed it through the audio (sync) queue while word-level
TTSTextFrames travel through the clock queue. The aggregation could reach
the assistant aggregator before the final words, leaving them orphaned
in the buffer. Stamp the frame with `_word_last_pts + 1` when there are
word timestamps so it can't overtake them.
- The aggregator's LLMAssistantPushAggregationFrame handler called
push_aggregation() directly, bypassing _trigger_assistant_turn_stopped.
For TTS-only flows there is no LLMFullResponseStartFrame, so the turn
start timestamp was never set and on_assistant_turn_stopped never fired.
Open a turn (if needed) and trigger stopped from the handler.
Fixes#4264.
f1a3ee97de