[RaphaelFakhri]

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What this PR does / why we need it:

Fixes #564.

WorkflowInstanceHandler persists runner logs from a System.Threading.Timer whose callback is an async void lambda. OnPersistLogBatchAsync had no exception handling, so any exception escaped the callback and terminated the operator process:

• A tick that runs while the handler is being disposed reads CancellationTokenSource.Token after disposal and throws ObjectDisposedException.
• A failure of ITextDocumentRepository.AppendAsync, for example a transient Redis error, also crashes the process.

This PR changes OnPersistLogBatchAsync to:

• Return early once the handler is disposed.
• Ignore ObjectDisposedException and OperationCanceledException, which only occur while the handler shuts down.
• Log any other exception instead of letting it escape the timer callback.
• Reset _persistingLogs in a finally block so the next tick persists the next batch.

Special notes for reviewers:

• The new WorkflowInstanceHandlerTests require a reference from Synapse.UnitTests to Synapse.Operator. That reference brings the Synapse.Operator namespace into scope, so MockSynapseApiClient now uses Synapse.Resources.Operator instead of Operator.
• Both new tests fail on main (ObjectDisposedException and IOException) and pass with this change.

Additional information (if needed):

A batch whose AppendAsync call fails is logged and dropped. The operator keeps running and the next batch is persisted.