Go SDK: answer the Dag-parsing request from Serve

[FrankYang0529]

Implement Go SDK: Answer the Dag-parsing request from Serve.

• Serve answers a DagFileParseRequest with one DagFileParsingResult. The result holds the Dags declared with airflow.Dag that the bundle registered, in registration order. The result leaves out the Dag of a task handler, because that Dag comes from a Python Dag file that is parsed separately.
• If a Dag fails to serialize, the result gets an import error that names that Dag.

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

• Yes - Claude Code

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• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
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• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[jason810496]

Pushed a few follow-up commits on top:

• Dropped the task-sdk and core stderr-excerpt changes, so this PR is Go SDK only. They can go in a separate PR if needed.
• Removed bundle.Registry. execution.Serve takes bundle.Bundle again and checks for bundle.DagSerializer in the parse branch. The adapter is now coordinatorSource{*taskHandlerMap; dags *dagMap}, the same shape as manifestSource in Go SDK: embed each native Dag's source file in the bundle #74342.
• The parse result now replies on the request id and exits, the same as the TS runtime and ADR 0003. The ack wait and its test are removed.
• parseDags returns an empty result when the bundle has no Dags, and logs Received Dag parse request / Parse-mode response at info, the same as TS.
• serializeRecovering now logs the panic stack.
• Trimmed the comments added in this PR.
• Native Dag task lookup in LookupTask will be fixed in Go SDK: embed each native Dag's source file in the bundle #74342.