Fix net472 build error: use TimeSpan.FromTicks for multiplication

Co-authored-by: halter73 <54385+halter73@users.noreply.github.com>

Author: Copilot

tests/ModelContextProtocol.Tests/Server/InMemoryMcpTaskStoreTests.cs

@@ -491,7 +491,7 @@ public async Task Dispose_StopsCleanupTimer()
         store.Dispose();
 
         // Advance time - timer should not fire after dispose
-        fakeTime.Advance(cleanupInterval * 3);
+        fakeTime.Advance(TimeSpan.FromTicks(cleanupInterval.Ticks * 3));
 
         // Assert - Store should still be accessible after dispose (no exceptions)
         // The cleanup timer should have stopped