oncetask: inject taskType into log records via ContextHandler

[anish749]

Problem

ContextHandler (oncetask/context_logger.go) currently auto-injects taskId and resourceKey from context but not taskType. When debugging across Cloud Logging, knowing the task kind is at least as useful as the task ID — it answers "what queue is this?" at a glance, lets you filter to a single handler's logs, and disambiguates errors when many task types share a resource key.

Solution

• Add a third context key, taskTypeContextKey, and have ContextHandler.Handle add taskType to every log record whose context carries it.
• Extend withTaskContext to take taskType string (unexported helper — small ripple). The two batch wrappers (withSingleTaskContext, withResourceKeyTaskContext) pass string(tasks[0].Type). All tasks in a resource-batch share the same Type, so a single value is correct for the batch.
• Update the one direct call site in once_task_firestore.go (cancellation loop).
• Update existing tests for the new signature; add TestContextHandler_InjectsTaskType for coverage.

taskType is structurally required on every task (it routes to the registered handler and keys the consumer loop), so it is always set unconditionally in withTaskContext — no empty-check needed.

Context

Follows on from #20. With handler errors now logged from SafeExecute using the per-task context, adding taskType to the auto-injected attributes makes those error records (and any handler-emitted logs) self-describing in Cloud Logging.