fix: revert AsyncDebounce.schedule to @sendable for Swift 6.0.2

CI on Xcode 16.2 (Swift 6.0.2) rejected the sending migration:
the sole caller (Transport.swift:107) passes a self-isolated closure
{ try await self.createAndSendOffer() } into Debounce (a separate
actor). Swift 6.0.2's region analyzer flags this as a strongly-
transferred-parameter race; 6.0.3+ accepts it but the project floor
is Xcode 16.2.

The cross-actor boundary (caller's actor → Debounce actor) is the
specific surface 6.0.2 scrutinizes more strictly. The other helpers
migrated in 82ffe16 don't cross actor boundaries the same way and
remain unaffected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: pblazej

Sources/LiveKit/Support/Async/AsyncDebounce.swift

@@ -32,7 +32,7 @@ actor Debounce {
         _task?.cancel()
     }
 
-    func schedule(_ action: sending @escaping () async throws -> Void) {
+    func schedule(_ action: @Sendable @escaping () async throws -> Void) {
         _task?.cancel()
         _task = Task.detached(priority: .utility) {
             try? await Task.sleep(nanoseconds: UInt64(self._delay * 1_000_000_000))