Phase 3: Implement GetUserMedia with full constraint handling

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Implement MediaDevices::GetUserMedia(MediaStreamConstraints). Handle both simple (bool audio/video) and advanced (detailed constraints) modes. Marshal constraints to native format. Create audio/video sources and return MediaStream with tracks.

Dependencies: Phase 2

Files to modify:

• WebRtcInterop/Media/MediaDevices.h/cpp

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Status update from merged PR #57 (#57):

Implemented in this phase:

• MediaDevices::GetUserMedia(...) now creates native local streams and adds requested audio/video tracks.
• Audio path uses CreateAudioSource(AudioOptions) + CreateAudioTrack(...).
• Video path now uses camera-backed capture (CameraVideoSource) with capability fallback startup tiers.
• Validation/error behavior improved for missing requested device kinds and track creation failures.

Still pending for this issue (full scope):

• Full marshalling and enforcement of detailed MediaStreamConstraints.AudioConstraints / VideoConstraints.
• Constraint-driven device/source selection semantics beyond bool-kind request handling.

Plan update:

1. Implement full constraints marshalling/application path in MediaDevices (kind + detailed constraints).
2. Align failure taxonomy with Media Capture expectation where constraints are unsatisfiable.
3. Add/extend interop tests around advanced constraint flows.

Related follow-ups:

• Implement MediaDevices.GetSupportedConstraints with real capability flags #58 (GetSupportedConstraints real flags)
• Implement InputDeviceInfo.GetCapabilities #59 (InputDeviceInfo.GetCapabilities)