Cross-SDK DagSerialization compatibility CI for the Go SDK #66942

Background

ADR 0003 explicitly promises:

The Go SDK gains a serde package that performs this encoding from
bundlev1.Bundle / bundlev1.Task, validated against
validation/serialization/test_dags.yaml (the same fixture set the
Java SDK uses), so the Go and Java outputs are byte-identical for
shared inputs.

The Go serde package exists
(go-sdk/pkg/execution/serde.go, serde_test.go) but the cross-SDK
parity hook does not - there is no prek/CI step that runs the
Go serializer against the shared test_dags.yaml and diffs the output
against Python's SerializedDAG.serialize_dag like the Java SDK does
via compare.py.

What needs to happen

1. Add a Go-side serializer driver that consumes
   validation/serialization/test_dags.yaml and emits a JSON file
   per Dag fixture (parallel to the Java
   SerializationCompatibilityTest).
2. Wire it into scripts/ci/prek/ so the existing compare.py runs
   against serialized_go.json alongside serialized_java.json.
3. Fail the check on any field-level divergence.

Acceptance criteria

• A regression in SerializedDAG.serialize_dag shape that Go doesn't
  follow fails CI before merge.
• The Go and Java SDKs share the fixture file (no Go-only copy).

Context

• ADR: go-sdk/adr/0003-coordinator-protocol-msgpack-ipc.md.
• Java parallel: Add CI, E2E Tests, and Pre-commit Hooks for Java SDK #65959 introduced
  check_java_serialization_compatibility.py; we extend the same
  pattern.
• Coordinator-distribution prek hooks tracking issue: Introduce common prek hooks for the new coordinator distribution #66515.

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