Use TimeProvider in InMemoryMcpTaskStore for deterministic testing

[Copilot]

Fixes flaky CleanupExpiredTasks_RemovesExpiredTasks test caused by timer-based cleanup relying on real time delays.

Changes

• InMemoryMcpTaskStore: Conditionally use ITimer (via TimeProvider.CreateTimer()) when MCP_TEST_TIME_PROVIDER is defined, falling back to Timer for production builds
• Tests: Replace Task.Delay() with FakeTimeProvider.Advance() in 5 time-dependent tests:
  • CleanupExpiredTasks_RemovesExpiredTasks
  • Dispose_StopsCleanupTimer
  • UpdateTaskStatusAsync_UpdatesLastUpdatedAt
  • CreateTaskAsync_MaxTasksPerSession_ExcludesExpiredTasks
  • ListTasksAsync_ConsistentWithExpiredTasksRemovedBetweenPages

Example

Before:

await Task.Delay(250, cancellationToken);  // Wait for TTL expiration and cleanup

After:

fakeTime.Advance(ttl + TimeSpan.FromMilliseconds(1));  // Expire task
fakeTime.Advance(cleanupInterval);  // Trigger cleanup timer

Tests are now deterministic and run >2 seconds faster.

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