Dag and task-level callbacks in the Go SDK

### Background `on_success_callback` / `on_failure_callback` / `on_retry_callback` / Dag-level `on_*_callback` are not addressable from a Go task today. The Java side is tracking the same gap under #66543. Probably blocked on a cross-language design decision (do we call back into Python, or do we invoke a separately-registered Go function via the coordinator protocol?). ### What needs to happen 1. Decide the callback shape jointly with the Java tracking issue (#66543) - probably a new coordinator-protocol message type. 2. Surface the registration on the Go side via `bundlev1.TaskSpec` / `DagSpec`. 3. Serialise the callback identity into the Dag JSON so the supervisor knows which Go function to dispatch back to. ### Acceptance criteria - A Go task can register `OnSuccess` / `OnFailure` / `OnRetry` callbacks that fire correctly under the executable-coordinator path. - Behaviour matches what the Java SDK ends up with under #66543. ### Context - Java parallel: #66543. - Recent registry work that touches `TaskSpec`: commit `857b0e1e0d8` ("Enhance task and DAG registration with optional specifications"). --- Drafted-by: Claude Code (Opus 4.7) (no human review before posting)