refactor(wfutils): one body behind Execute and ExecuteWithLowPrioQueue

[KillerX]

12/n of a stack. Base: refactor/remove-dynamic-trigger-rpc (#450). Closes ExecuteWithLowPrioQueue has drifted from Execute.

The two differed only in the queue they pick, but each had its own copy of the scheduling code and the copies had drifted. Two of the three divergences the audit listed are already gone earlier in this stack — the ActivityWG bookkeeping no longer exists (#440) and both call activityOptionsWithDefaults (#443) — leaving the duplicated retry-policy switch. Both now call executeOnQueue with a queue.

Extracting it exposed a gap the duplication hid. The switch only names the worker, transcode and audio queues, so an activity on any other queue got no retry policy at all unless the workflow set one — and low-priority is by definition a queue the switch does not name, so ExecuteWithLowPrioQueue routed activities to exactly the case that falls through. Latent today because both callers pass GetDefaultActivityOptions, which carries a policy. retryPolicyForQueue now has a default branch: low priority and live ingest carry the same kind of work as the worker queue, so they get the same loose policy.

Two details worth keeping in mind when reading the diff, both about debug mode:

• The low-priority remap compares against environment.QueueWorker, the constant, not GetWorkerQueue(). With QUEUE=debug the accessor returns the debug queue and the debug worker polls only that queue, so remapping there would leave the activity unscheduled. The asymmetry is deliberate.
• Debug also collapses the worker, transcode and audio accessors onto one value, which made the original a switch with three identical cases where the first won. The extracted version keeps worker first for that reason, and retryPolicyForQueueNames takes the names as arguments so a test can exercise the collapsed arrangement — the QUEUE variable that decides them is read at package-var time, so t.Setenv cannot reach it.

Worth knowing while reviewing: the test environment here runs with QUEUE=debug, so anything asserting on the accessors is asserting the collapsed arrangement.

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