oncetask: log handler errors from SafeExecute

[anish749]

Problem

When a task handler returns an error, oncetask records the error in the task's errors array in Firestore but does not emit a log line. Handler-error visibility in Cloud Logging therefore depends on each handler logging the error itself. The two completion-adjacent log lines (once_task_firestore.go:292, :319) only fire when the Firestore persistence of the completion itself fails — a different, rarer event — and they use m.ctx, so they don't pick up taskId/resourceKey (or any caller-attached fields) via ContextHandler.

Solution

Emit a single slog.ErrorContext(ctx, "Handler returned error", "error", err) inside SafeExecute when the wrapped function returns an error. SafeExecute is the chokepoint for all three production handler paths (cancellation, single-task, resource-batch). The ctx passed in already carries taskId/resourceKey via withTaskContext / withSingleTaskContext / withResourceKeyTaskContext, so ContextHandler (oncetask/context_logger.go) auto-injects them and any caller-attached fields ride along.

Context

• Recurrence-task bookkeeping (filterAndSpawnOccurrences) doesn't invoke user handlers and already logs its own errors.
• The "claimed multiple tasks" invariant check at once_task_firestore.go:307-308 is an oncetask-internal bug and already logs itself.
• Panic recovery in SafeExecute continues to log at Error with the same per-task ctx.