fix: implement stop audio functionality in Bidi demo

[42nick]

Fixes #795

Fix: Implement Stop-Audio Functionality in BiDi Demo

This pull request adds support for stopping ongoing audio playback in the BiDi (bidirectional audio) demo.

What’s changed

• Implements a Stop Audio action so users can interrupt an active voice interaction after it has started.
• Updates the BiDi demo UI and control flow to properly handle stopping an audio stream that was previously only startable.
• Fixes a bug where audio playback could not be halted once triggered.

Why this matters

• Interactive audio demos should allow users to cancel voice output without waiting for playback to complete.
• Improves developer and user experience when testing conversational and real-time audio workflows in the ADK samples.

Related context

• Addresses the reported issue in the BiDi demo where stop-audio functionality was missing.

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