Use TimeProvider in InMemoryMcpTaskStore for deterministic testing

[Copilot]

Plan: Add TimeProvider Support to InMemoryMcpTaskStore ✅

• Update InMemoryMcpTaskStore to use TimeProvider instead of System.Threading.Timer
  • Replace Timer? with ITimer? field (conditionally compiled)
  • Update constructor to accept optional TimeProvider parameter (test-only)
  • Use timeProvider.CreateTimer() instead of new Timer()
• Update tests to use FakeTimeProvider and remove Task.Delay calls
  • Update CleanupExpiredTasks_RemovesExpiredTasks to use FakeTimeProvider
  • Update Dispose_StopsCleanupTimer to use FakeTimeProvider
  • Update UpdateTaskStatusAsync_UpdatesLastUpdatedAt to use FakeTimeProvider
  • Update CreateTaskAsync_MaxTasksPerSession_ExcludesExpiredTasks to use FakeTimeProvider
  • Update ListTasksAsync_ConsistentWithExpiredTasksRemovedBetweenPages to use FakeTimeProvider
• Run full test suite to ensure no regressions (All 1562 tests passed)
• Run security scanning (No alerts found)
• Fix net472 compatibility issue: Replace cleanupInterval * 3 with TimeSpan.FromTicks(cleanupInterval.Ticks * 3)

Summary

Successfully updated InMemoryMcpTaskStore to use TimeProvider for deterministic, fast testing. All tests that previously used Task.Delay now use FakeTimeProvider.Advance() to simulate time passage, making them:

• Deterministic: No more timing-related flakiness
• Fast: Tests complete instantly instead of waiting for real time delays
• Maintainable: Clear intent through explicit time advancement
• Compatible: Works with all target frameworks including net472

The changes are backward compatible with production code (using conditional compilation) and all tests pass successfully.

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