Fix 288ms UI hang in video publishing flow

[sampepose]

🔗 Issue Links

N/A - Performance improvement identified through profiling

🎯 Goal

Fix a 288ms UI hang that occurs when publishing video, caused by synchronous camera initialization blocking the main thread.

📝 Summary

• Eliminated 288ms UI hang when enabling video by making camera startup asynchronous
• Used Task.detached to move blocking RTCCameraVideoCapturer.startCapture() off the critical path
• No functional changes - camera still initializes, just doesn't block UI
• Faster video publishing - transceivers set up immediately while camera starts in parallel

🛠 Implementation

Instruments profiling revealed that RTCCameraVideoCapturer.startCapture() (Google's WebRTC code) was blocking the main thread for 288ms with a __psynch_cvwait during camera initialization.

Root cause: The video publishing flow was waiting synchronously for the camera to start before continuing with transceiver setup.

Solution: Wrap the startVideoCapturingSession() call in a Task.detached to allow camera initialization to happen in parallel while the rest of the publish flow continues immediately.

// Camera starts async, doesn't block publish flow
Task.detached { [weak self] in
    do {
        try await self?.startVideoCapturingSession()
    } catch {
        log.error("Failed to start camera: \(error)")
    }
}

This approach ensures the UI remains responsive while the camera hardware initializes in the background.

🎨 Showcase

Before	After
288ms hang when enabling video	0ms - instant response
Publishing blocked by camera init	Publishing proceeds immediately
UI freezes during camera startup	Smooth 60fps maintained

🧪 Manual Testing Notes

1. Join a call with video disabled
2. Enable video during the call
3. Verify no UI hang when toggling video on
4. Confirm video appears once camera initializes (~300ms later)
5. Test rapid video on/off toggling
6. Verify camera position changes (front/back) remain smooth
7. Check that video quality is unaffected

☑️ Contributor Checklist

• I have signed the [Stream CLA](https://docs.google.com/forms/d/e/1FAIpQLScFKsKkAJI7mhCr7K9rEIOpqIDThrWxuvxnwUq2XkHyG154vQ/viewform) (required)
• This change follows zero ⚠️ policy (required)
• This change should receive manual QA
• Changelog is updated with client-facing changes
• New code is covered by unit tests
• Comparison screenshots added for visual changes
• Affected documentation updated (tutorial, CMS)

[ipavlidakis]

Hey @sampepose, thank you for this PR. We are currently in the process of resolving many performance issues (you can take see a sneak peek in the open PRs).

We are now in a state where we have resolved all the hungs and for that reason I'm not going to accept this PR.

I'm going to keep open though as I reminder to check again once the changes have been merged.

Thanks,
Ilias

[sampepose]

Hey! I noticed #829 still has this bug. It's blocking MainActor in LocalAudioMediaAdapter.swift