Stage 2: Add optional UniTask surface behind LIVEKIT_UNITASK

[MaxHeimbrock]

Summary

• New LiveKit.UniTask asmdef hosts an AsUniTask extension on YieldInstruction and StreamYieldInstruction.
• Asmdef compiles only when com.cysharp.unitask is installed — versionDefine auto-activates LIVEKIT_UNITASK. When UniTask is absent, the extension simply does not exist; no compile error, no runtime cost.
• AsUniTask(CancellationToken ct = default) wraps the Stage 1 awaiter in a UniTaskCompletionSource and adds cancellation with abandon-awaiter semantics — a cancel faults the UniTask with OperationCanceledException but the underlying FFI request is not aborted. Wire-level cancellation is deferred.
• GetResult stays non-throwing for IsError parity with yield return / await (callers still inspect IsError). Throwing variants can be layered on later.

Sample migration

Samples~/Meet/Assets/Runtime/MeetManager.cs is migrated end-to-end to demonstrate the path: ConnectToRoom, PublishLocalCamera, PublishLocalMicrophone all switch to async UniTask with cancellation bound to this.GetCancellationTokenOnDestroy(). Event handlers stay Action-compatible (void) and fire-and-forget via .Forget(). Long-running per-frame pumps (source.Update(), stream.Update()) stay on StartCoroutine since they are not request/response.

Testing

• Unit tests (Tests/PlayMode/UniTask/RoomUniTaskTests.cs) — gated by the same LIVEKIT_UNITASK define:
  • AsUniTask_CompletesOnIsDone — synthetic instruction, verifies UniTask resumes when IsDone is set and IsError is observable on resume.
  • AsUniTask_Cancellation_ThrowsOperationCanceled — synthetic instruction, verifies cancellation throws OperationCanceledException and the underlying instruction is untouched.
• An additional E2E test against the real FFI Connect path was tried and removed: FFI error logs arrive asynchronously and their delivery window races UniTask's synchronous resume, so LogAssert tracking was brittle across test order. The unit tests cover the extension's logic; end-to-end coverage is now the migrated Meet sample.

Targeting

Base branch is max/yield-instruction-getawaiter (Stage 1 PR #289), so this diff shows only the Stage 2 surface area.

Test plan

• Scripts~/run_unity.sh build macos — clean compile with UniTask restored via UPM git URL.
• All 4 PlayMode tests pass (Connect_FailsWithInvalidUrl|AsUniTask filter): 4 passed, 0 failed.
• Reviewer: run livekit-server --dev, then open Samples~/Meet and verify Start Call / Camera / Mic flows all work end-to-end on UniTask.
• Reviewer: confirm that omitting UniTask from the manifest (e.g. for Samples~/Basic) does not produce a compile error — LiveKit.UniTask asmdef should be cleanly skipped.

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