### Summary
`AsyncExecutor.Execute` acquires a worker-pool slot, while
`ExecuteWithRetry` duplicated the task lifecycle without acquiring one.
Pregel supplies a retry configuration for every node, so those
executions
bypassed `WithMaxConcurrency`.
This change:
- routes retry execution through the shared `Execute` path;
- waits on the task-owned context and rechecks cancellation after slot
acquisition;
- adds deterministic regression coverage for worker-pool occupancy and
queued-task cancellation.