fix(voice): clear unplayed realtime messages from chat ctx after interrupt

[chakshu-dhannawat]

Problem

Fixes #6424. When a realtime response produces multiple messages (e.g. a gpt-realtime preamble plus the final answer) and the user interrupts after message A has finished playing but before message B is pulled from message_stream, B is never played and never committed to the local chat context, but it stays in the server-side conversation. On the next turn the model believes it already said B and references an answer the user never heard.

Root cause

AgentActivity._realtime_generation already intends to handle this. _process_messages deliberately leaves unconsumed messages out of message_outputs:

async for msg in generation_ev.message_stream:
    if speech_handle.interrupted:
        break            # remaining messages left out of message_outputs
    entry = await _process_one_message(msg)
    message_outputs.append(entry)
    if entry.out.played == "partial":
        break

But the cleanup is guarded on any_skipped, which is computed by scanning message_outputs — the very list those messages were left out of:

if speech_handle.interrupted and any_skipped and self.llm.capabilities.mutable_chat_context:
    await self._rt_session.update_chat_ctx(self._agent._chat_ctx)

So for the exact case the comment describes ("interrupted before we pulled them"), any_skipped is always False and update_chat_ctx() never runs. The window where any_skipped becomes True (a message pulled into message_outputs, forwarded, and played zero frames) is very narrow, so in practice the guard is inverted relative to its intent.

Fix

Track the abandon condition where it happens instead of inferring it from message_outputs: set a messages_abandoned flag in _process_messages when we break out on an interrupt or a partial play, and run the cleanup on (any_skipped or messages_abandoned). update_chat_ctx is idempotent, so in the rare case where the interrupted message happened to be the last one of the response this performs one harmless extra sync.

This matches the direction suggested in the issue (which the reporter verified locally, and a maintainer green-lit).

Testing

Added tests/test_realtime_interrupt_chat_ctx.py, a hermetic unit test (no network) that reproduces the interrupt-between-messages race with a fake realtime model whose session commits conversation items in generate_reply() the way a real server does. It asserts the never-played message does not survive server-side. The test fails before this change and passes after. ruff check, ruff format, and mypy -p livekit.agents are clean; the existing --unit suite still passes (the only failures locally are pre-existing tests/test_room.py cases unrelated to this change).

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.