Go SDK: embed each native Dag's source file in the bundle

[jason810496]

• related: TS SDK: embed one source region per native Dag file #73723, ADR-0015 (airflow-core/adr/lang-sdk/0015-per-dag-source-in-bundle-artifact.md)
• Depends on Parse native Go Dags and show each Dag's own source #74341 get merged first: it defines the source layout in executable-bundle-spec.rst and reads it.

Why

A Go bundle embedded only the func main file, so a native Dag declared in another file or package had no source of its own to show in the Code view. Also, Serve looked tasks up among the task handlers only, so the tasks of a native Go Dag could not run.

How

• airflow.Dag records the file that calls it, and --airflow-metadata reports it per native Dag:

  // dags/reports.go
  func Reports() *airflow.DagRef {
      dag := airflow.Dag("reports") // recorded as dags/reports.go
      dag.Task(publish)
      return dag
  }

• airflow-go-pack embeds the entrypoint and each Dag's file once, as paths relative to the module root. A Dag declared in a file outside the module maps to the entrypoint, with a warning.

  entrypoint_path: "example/bundle/main.go"
  dag_source_paths:
    orders: "example/bundle/main.go"
    reports: "example/bundle/dags/reports.go"
  sources:
    - {path: "example/bundle/main.go", offset: 0, length: 1532, sha256: "..."}
    - {path: "example/bundle/dags/reports.go", offset: 1532, length: 811, sha256: "..."}

What

• airflow.Dag records its caller's file. Serve --airflow-metadata lists the files under dag_source_files; task handlers add nothing. The files come through a new DagSourceLister interface, separate from the task-handler EnumerableBundle.
• airflow-go-pack:
  • writes entrypoint_path, dag_source_paths and sources in place of source, in a stable order, so identical inputs give a byte-identical bundle;
  • accepts absolute and -trimpath build paths;
  • packs a bundle that only has native Dags.
• airflow-go-pack inspect --source prints each embedded file under its path.
• Serve finds the tasks of Dags built with airflow.Dag as well as task handlers, so a task of a native Go Dag runs instead of being reported as not registered. go-sdk/internal/bundle/doc.go says so.
• Docs: go-sdk/README.md, go.rst and the example Justfile.

Not in scope

• The dags inventory of dag_ids and task_ids in the bundle metadata is unchanged, and a mixed-language task still finds its bundle through it. Dropping it, as ADR-0014 proposes, is left to a follow-up, which then only deletes EnumerableBundle and its use.
• The packer still runs the binary with --airflow-metadata, now also to learn each Dag's source file.

---

Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 5.5 following the guidelines

[henry3260]

Just want to help unblock the technical side here, not a formal review.

[jason810496]

Thanks for the review.