Adds five tests for the delayed-transcript flow on
`LLMUserAggregator`:
- basic flow: `on_user_turn_stopped` fires fast with empty content;
`on_user_turn_message_finalized` fires later with the populated
transcript; user message lands in context.
- backstop with no transcript: backstop timer still finalizes the
turn; message_finalized fires with empty content; no user message
added to context.
- next-turn precondition violation: a new VAD start fires while the
previous turn is still pending; the previous turn is force-flushed
before the new turn begins.
- context-order with assistant response: paired aggregators with a
late user transcript arriving before the assistant content streams;
verifies the user message lands in context before the assistant
message (the conversational-order invariant the design relies on).
- strategy mutation: explicit start/stop strategies are mutated by
the bundle — `TranscriptionUserTurnStartStrategy` is dropped from
start, `wait_for_transcript=False` is flipped on the stop strategy
that had it explicitly set to True.
Tests patch `DEFAULT_TTFS_P99` to keep the backstop fast.
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