When targetTaskId is passed, the lease attempt must target that task only.
The previous code silently fell back to queue-head selection if the targeted
task was not available — producing a lease for a different task, then
returning it with the wrong taskId. acquireReuseHandoff would then see
lease.taskId !== input.task.id and emit a misleading 'no-lease' diagnostic
that hid the real cause (queue head was occupied by an unrelated task).
The fix: when targetTaskId is set and the lease for that task returns null,
return null immediately instead of falling through to the queue-head SELECT.
Callers (acquireReuseHandoff) already validate the returned taskId and emit
structured 'no-lease' diagnostics including acquiredTaskId so the polluter
is identifiable in audit logs.
Queue-head fallback is preserved for callers that omit targetTaskId
(backward-compatible).
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