SyncActivity replication returns nil instead of RetryReplication error when workflow not found — retry mechanism not triggered

Summary

When SyncActivity replication cannot find the target workflow,
it returns nil (success) instead of
serviceerrors.NewRetryReplication. This means the retry/resend
mechanism is never triggered for this case.

Root Cause

activity_state_replicator.go:214:

if _, isNotFound := err.(*serviceerror.NotFound); isNotFound {
    // TODO: this should return serviceerrors.NewRetryReplication
    // resend logic will handle not found case and drop the task.
    return nil  ← silently drops the task
}

Impact

• Replication tasks are silently dropped when workflow not found
• Retry/resend logic never gets a chance to handle the case
• Could cause missed replication events without any error signal

Suggested Fix

Return serviceerrors.NewRetryReplication to trigger the resend
path, which already handles the not-found case correctly.

References

• service/history/ndc/activity_state_replicator.go:214