fix(voice): keep the agent thinking while a tool is still running

[VihaanAgarwal]

with_filler speech during a long tool call knocks the session back to listening, which arms the user-away timer while the tool is still running. With user_away_timeout set, the user gets marked away mid-lookup and the app's inactivity handling fires over the agent's own tool call.

Two paths reset the state when a speech ends, and neither knows a tool is in flight:

• _tts_task_impl sets listening when the filler finishes playing
• _on_pipeline_reply_done does the same as the say task's done callback

The pipeline reply has the same gap without any filler: at playout end it only picks thinking when a tool already finished (len(tool_output.output) > 0), so a slow tool drops the state to listening until its result comes back.

The fix consults in-flight tool executions at those three spots. _background_speeches already tracks speeches that are awaiting their tools, so the two speech-end paths hand back thinking when it's non-empty. The reply task also stays in thinking when exe_task hasn't completed yet.

Added a test that runs a slow tool with a filler and a short user_away_timeout. On main the filler ends in listening and the user flips to away mid-tool. With the fix both speech ends return to thinking and the away timer stays disarmed. Unit suite, ruff and mypy are green.

Fixes #6904

[devin-ai-integration]

✅ Devin Review: No Issues Found

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

View in Devin Review to see 2 additional findings.

[longcw]

looks good to me, but one thing I am not sure is

[longcw]

this branch also calls _on_end_of_agent_speech and _restore_interruption_by_audio_activity

[longcw]

maybe move it to the other branch, but may need to take care if there is no tool reply after exe_task, the agent state should be back to listening

elif self._session.agent_state == "speaking":
    # a tool still running keeps the agent busy: "listening" here would arm the
    # user-away timer mid-tool (#6904)
    still_running = not speech_handle.interrupted and not exe_task.done()
    self._session._update_agent_state("thinking" if still_running else "listening")