fix(voice): resolve video task leak in AgentSession

[Mayank-MSJ-Singh]

I was just checking this project and saw this, did some test and felt it might be a real bug, so i fixed it and created this PR!

Description

This change resolves a resource leak where the video forwarding task (_forward_video_atask) was left dangling in the background on session teardown.

The Problem

During session teardown (AgentSession._aclose_impl), the cleanup sequence explicitly cancels and awaits the audio forwarding task (_forward_audio_atask), but completely omitted _forward_video_atask.

This omission leads to leaks under two scenarios:

1. Early Abort (No Activity): If a session starts and is aborted before an activity is initialized (so self._activity is None), the block detaching the inputs is skipped. The active video task is never cancelled and continues running indefinitely.
2. Clean Teardown: Even when an activity is present, setting self.input.video = None cancels the active task but immediately schedules a new redundant task. Explicitly awaiting the task ensures the event loop is cleanly purged of any scheduled forwarding tasks.

The Fix

Added symmetric cancellation and cleanup in AgentSession._aclose_impl matching the audio task's handling:

if self._forward_video_atask is not None:
    await utils.aio.cancel_and_wait(self._forward_video_atask)

[CLAassistant]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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