Changed catch (TaskCanceledException) to
catch (OperationCanceledException) when (!cancellationToken.IsCancellationRequested)
so that only internal debounce timer resets are silently ignored, while
external CancellationToken cancellations (e.g. test timeouts, caller-driven
cancellation) correctly propagate to the caller.

Also replaced the reassignment of the `cancellationToken` parameter with a
separate `internalToken` variable to preserve the original external token
for use in the exception filter. Removed the outdated <remarks> stating
that exceptions are swallowed.

Refine the OperationCanceledException filter to explicitly check both
conditions: internalToken.IsCancellationRequested (the debounce timer
was reset by a new RunAsync call) AND !cancellationToken.IsCancellationRequested
(the external token was NOT cancelled). This makes the intent clear and
guards against edge cases where the exception could have a different origin.