Replace BasePipelineTask with BaseTask
Drops the old abstract `BasePipelineTask` and replaces it with `BaseTask` — the common base for any runtime task. `BaseTask` subscribes to a `TaskBus`, participates in the shared `TaskRegistry`, handles activation / deactivation, end / cancel, and the full `@job` RPC surface (request_job, job, job_group, send_job_response / update / stream_*, etc.). It ships a default `run()` for bus-only tasks; subclasses with their own runtime (e.g. `PipelineTask`) override it.
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