Stabilize SystemFolder watcher tests

[Arlodotexe]

Background

OwlCore.Storage includes integration tests for the SystemFolder watcher built on FileSystemWatcher. These tests validate that create/delete events are surfaced via IFolderWatcher.CollectionChanged.

Problem

• The full test run intermittently crashed with:
  • InvalidOperationException: “An attempt was made to transition a task to a final state when it had already completed.” This was caused by TaskCompletionSource.SetResult being invoked multiple times due to duplicate/rapid FileSystemWatcher events.
  • UnauthorizedAccessException when attempting to create files/folders directly under %TEMP%, and occasional name collisions from predictable test item names.

Solution

• Stabilized watcher tests in tests/OwlCore.Storage.Tests/SystemIO/SystemFolderTests.cs:
  • Create a unique temp subfolder per test run (%TEMP%\{Guid}) to avoid permission issues and cross-test interference.
  • Replace TaskCompletionSource.SetResult with TrySetResult and use TaskCreationOptions.RunContinuationsAsynchronously to safely handle duplicate events without deadlocks.
  • Use unique, GUID-based file/folder names to avoid collisions.
  • Add timeout of 2000ms for filesystem/event variability.
• No product code or public APIs changed; this is a tests-only stabilization.
• Result: Full suite passes locally (328/328) succeeded on net8.0,

[Copilot]

Pull Request Overview

This PR stabilizes flaky SystemFolder watcher tests by addressing race conditions and filesystem permission issues that were causing intermittent test failures.

• Creates unique temporary directories per test run to avoid permission issues and cross-test interference
• Replaces unsafe TaskCompletionSource.SetResult calls with TrySetResult and proper async continuation options to handle duplicate filesystem events
• Uses GUID-based names for test files/folders to prevent naming collisions

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