[camera_web] Skip video input devices that cannot be started in availableCameras

[Anfet]

availableCameras opens every video input device to read its facing mode. If one device cannot be started (for example the OBS Virtual Camera on Windows while OBS is not running), getUserMedia rejects with NotReadableError and the whole call throws cameraNotReadable, so the cameras that work are not returned either.

This PR skips a device whose stream cannot be started and continues with the remaining devices. If no camera is available and a device failed, the first device error is still thrown, so a single broken camera keeps reporting its error instead of returning an empty list.

The platform interface documents that availableCameras returns an empty list when no cameras are available. availableCameras already throws today when a device cannot be started, and this PR keeps that behavior when no device can be started at all, so a single busy or broken camera still reports why it failed. I'm happy to return an empty list instead if that is preferred.

Fixes flutter/flutter#194184

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Code Review

This pull request updates the camera_web package to version 0.3.5+7. The availableCameras method in CameraPlugin is modified to catch CameraWebException when retrieving the video stream for a device, allowing other functional cameras to be listed instead of failing the entire call. If no cameras are successfully retrieved, the first caught exception is thrown. Integration tests are added to verify these scenarios. There are no review comments, and there is no feedback to provide.