Fix Unity media stream lifetime leaks and add latest-frame video coalescing

[xianshijing-lk]

This PR tightens Unity-side media stream lifetime management and reduces unnecessary video frame retention under bursty delivery.

What changed

• Fixed AudioStream cleanup so it fully releases owned resources on dispose:
  • unsubscribe from FfiClient.AudioStreamEventReceived
  • unsubscribe/destroy AudioProbe
  • dispose RingBuffer
  • dispose AudioResampler
  • dispose the owned FFI stream handle
• Fixed per-callback audio frame wrapper leaks by disposing both:
  • the incoming AudioFrame
  • the remixed/resampled output AudioFrame
• Made AudioResampler explicitly disposable and responsible for releasing its native resampler handle.
• Fixed VideoStream cleanup so it:
  • unsubscribes from FfiClient.VideoStreamEventReceived
  • disposes current and pending native frame buffers
  • disposes the owned FFI stream handle
• Reworked VideoStream buffering to latest-frame-wins:
  • separate pending vs currently consumed frame buffers
  • coalesce bursts to the newest pending frame
  • drop superseded pending frames immediately
  • drop incoming frames immediately when the stream is not actively playing
  • dispose the currently displayed native frame buffer on Stop()
• Added comments documenting the producer/consumer split between _pendingBuffer and VideoBuffer.
• Added EditMode regression tests covering media stream disposal and video coalescing invariants.

Why
Before this change, Unity media streams could retain stale event subscriptions and native resources across unsubscribe/disconnect/reconnect flows. AudioStream and VideoStream subscribed to long-lived FfiClient events but did not fully
tear down owned resources, and audio callback-scoped frame wrappers were not being disposed.

For video, Unity also only had a single active buffer slot, which was less explicit about producer/consumer separation and did not document/latest-frame coalescing behavior clearly. This PR makes that behavior explicit and ensures
older pending frames are dropped immediately rather than retained.

Behavioral impact

• No public API changes.
• Under video burst/backlog conditions, Unity now more explicitly keeps only the newest pending frame for upload.
• If VideoStream is not actively playing, incoming video frames are dropped immediately on the Unity side instead of being retained.
• Media stream disposal is stricter and releases native resources sooner.

Testing

• Added EditMode regression tests for media stream cleanup and latest-frame coalescing structure.
• I could not run a build in this shell because dotnet / msbuild are not available here.

[cloudwebrtc]

lgtm!