[VoiceLive] Harden async samples for GA: fix threading races and reactor stream issues

[xitzhang]

Description

Addresses reviewer feedback before GA:

"The async samples have some issues that should be addressed before GA. Customers using these samples in production can run into various threading issues and can also lead to certain events not getting processed."

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This PR hardens the VoiceLive Java async samples ahead of GA by addressing common race conditions in audio processing and improving reactive subscriptions/error handling, while also shifting sample authentication guidance toward DefaultAzureCredential (with API key as an option).

Changes:

• Update samples to use DefaultAzureCredentialBuilder by default (API key becomes optional / flag-driven where applicable).
• Reduce concurrency hazards in audio capture/playback by adding volatile fields and bounding playback queues with drop warnings.
• Improve reactive subscribe() usage by providing explicit error consumers and restructuring some event pipelines.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/VoiceAssistantSample.java	Switches auth default to DefaultAzureCredential; adds volatile audio lines and bounded playback queue; updates CLI/env var guidance (needs doc fixes).
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/telemetry/GlobalTracingSample.java	Uses DefaultAzureCredential by default and updates env var requirements accordingly.
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/MicrophoneInputSample.java	Uses DefaultAzureCredential by default and updates env var requirements accordingly.
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/MCPSample.java	Uses DefaultAzureCredential; bounds playback queue; adds subscribe error handlers (but introduces a blocking playback-thread shutdown issue).
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/FunctionCallingSample.java	Uses DefaultAzureCredential; bounds playback queue; adds subscribe error handlers (but introduces a blocking playback-thread shutdown issue).
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/BasicVoiceConversationSample.java	Uses DefaultAzureCredential by default and updates env var requirements accordingly.
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/AudioPlaybackSample.java	Uses DefaultAzureCredential; bounds audio queue; changes receiveEvents timing and adds a receiveSink-related comment (needs adjustment to avoid relying on internals).
sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/AgentV2Sample.java	Makes session/audio processor references volatile; improves subscribe error handling; bounds playback queue and adds drop warnings.

Comments suppressed due to low confidence (2)

sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/MCPSample.java:554

• startPlayback() now blocks on playbackQueue.take(), but cleanup() only sets isPlaying=false and closes the speaker. If the queue is empty, the playback thread can block indefinitely and won’t observe isPlaying=false, so playback may not stop cleanly. Consider sending a shutdown marker into the queue and/or keeping a thread reference and interrupting it (and/or mark the thread daemon).

                // Start playback thread
                new Thread(() -> {
                    while (isPlaying.get()) {
                        try {
                            byte[] audioData = playbackQueue.take();
                            speaker.write(audioData, 0, audioData.length);
                        } catch (InterruptedException e) {
                            Thread.currentThread().interrupt();
                            break;
                        }
                    }
                }, "AudioPlayback").start();

sdk/voicelive/azure-ai-voicelive/src/samples/java/com/azure/ai/voicelive/FunctionCallingSample.java:539

• startPlayback() now blocks on playbackQueue.take(), but cleanup() only flips isPlaying=false and clears the queue. If the queue is empty at shutdown, the playback thread can block indefinitely and won’t exit. Consider enqueuing a sentinel/shutdown item during cleanup and/or keeping a thread reference and interrupting it (and/or mark the thread daemon).

                // Start playback thread
                new Thread(() -> {
                    while (isPlaying.get()) {
                        try {
                            byte[] audioData = playbackQueue.take();
                            speaker.write(audioData, 0, audioData.length);
                        } catch (InterruptedException e) {
                            Thread.currentThread().interrupt();
                            break;
                        }
                    }
                }, "AudioPlayback").start();