Fix crash on rapid double-clicking video record button

[davidjiagoogle]

Summary

Fixes an issue (b/535645067) where double-clicking or rapidly tapping the video recording button in quick succession crashed the app with IllegalStateException: A recording is already in progress.

Changes Made

1. Added CaptureButtonUiState.Enabled.Recording.Pending: Created a new UI state representing the initialization window when video recording is starting.
2. Mapped VideoRecordingState.Starting: Mapped VideoRecordingState.Starting to CaptureButtonUiState.Enabled.Recording.Pending instead of Idle in CaptureButtonUiStateAdapter.kt.
3. Updated CaptureButtonComponents.kt: Added Recording.Pending to the ignored/no-op states during click handling (onKeyUp), preventing rapid consecutive clicks from sending duplicate StartRecordingEvents.
4. Defensive Error Handling in CameraSession.kt: Replaced throwing an unhandled IllegalStateException with logging a warning when receiving a duplicate StartRecordingEvent during active recording.
5. Unit Tests: Updated CaptureButtonUiStateAdapterTest.kt to assert the new Pending state.

[gemini-code-assist]

Code Review

This pull request introduces a new Pending state to CaptureButtonUiState.Enabled.Recording to represent when a video recording request has been sent but is still initializing. This state is mapped from VideoRecordingState.Starting in the adapter and handled in the capture button components and tests. Additionally, CameraSession now logs a warning instead of throwing an exception if a start recording event is received while a recording is already in progress. Feedback was provided regarding a potential bug where releasing the capture button during the Pending state would ignore the key-up event, causing the recording to continue indefinitely; a code suggestion was provided to handle this state in onKeyUp.

[temcguir]

It would be awesome to add a quick unit test to PreviewViewModelTest.kt to ensure this specific fast-release cancellation doesn't regress.

First, add a diagnostic counter to FakeCameraSystem.kt:

    var numVideoRecordingStarts = 0
    
    override suspend fun startVideoRecording(
        saveLocation: SaveLocation,
        onVideoRecord: (OnVideoRecordEvent) -> Unit
    ) {
        if (!useCasesBinded) {
            throw IllegalStateException("Usecases not bound")
        }
        numVideoRecordingStarts++
        recordingInProgress = true
    }

Then we can add the following test directly inside PreviewViewModelTest.kt to verify that rapidly firing "stop" immediately cancels the "start" job:

    @Test
    fun fastStartAndStopVideoRecording_cancelsRecording() = runTest(StandardTestDispatcher()) {
        startCameraUntilRunning()
        
        // Start and stop immediately without advancing the dispatcher
        previewViewModel.captureController.startVideoRecording()
        previewViewModel.captureController.stopVideoRecording()
        
        // Let the coroutines execute
        advanceUntilIdle()
        
        // Verify that because we cancelled the job synchronously, the start implementation
        // was bypassed completely to protect us from the channel race condition!
        assertThat(cameraSystem.numVideoRecordingStarts).isEqualTo(0)
    }