Dag and task-level callbacks in the Go SDK

[jason810496]

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
   (Java-based task and dag level callbacks #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 Java-based task and dag level callbacks #66543.

Context

• Java parallel: Java-based task and dag level callbacks #66543.
• Recent registry work that touches TaskSpec: commit 857b0e1e0d8
  ("Enhance task and DAG registration with optional specifications").

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Drafted-by: Claude Code (Opus 4.7) (no human review before posting)