Probe a Lang-SDK artifact for its task handlers

[jason810496]

Stack (bottom to top), on native #74037 (stack #74170): #73973, #73974, #73975, #74317, #73976, #74067, #74135, #74140, #74141, #73971, #74030, #74031, #74032, #74136, #74137, #74138, #74139

To check stub tasks, the Dag processor has to find the artifact a worker would run, start its runtime inside the Dag-parsing child, and ask it with #73973's messages. This PR adds that probe and the two coordinator hooks it needs. Nothing calls it yet: #73975, #74317 and #73976 implement the hooks for Go, Java and TypeScript, and #74135 calls the probe.

class MyCoordinator(SubprocessCoordinator):
    def _find_task_handler_artifact(self, *, bundle_path, dag_id):
        # The artifact a stub task of dag_id runs, found the way execute_task finds it.
        return ResolvedBundle(bundle_path / "handlers", "2026-10-30")

    def _build_parse_task_handler_command(self, *, path):
        # The command without --comm / --logs, and the runtime's supervisor schema version.
        return [os.fspath(path)], "2026-10-30"


result = LangSDKTaskHandlerProcessorProcess.run(
    coordinator="my-sdk",  # key in [sdk] coordinators
    path=bundle_path / "handlers",
    bundle_path=bundle_path,
    bundle_name="task-handlers",
    artifact_rel_path="handlers",
    logger=log,
    deadline=time.monotonic() + 30,  # optional
)  # every handler the artifact registers; a failure is in result.import_errors

• The hooks live on SubprocessCoordinator only, and BaseCoordinator is unchanged. A coordinator without them gives an import error that says so.
• Only a runtime that reports supervisor schema 2026-10-30 or later is probed. That is the in-progress version, so no new date is invented.
• LangSDKTaskHandlerProcessorProcess is a standalone process class beside Parse native Lang-SDK Dags in the Dag processor with their runtime #74035's LangSDKDagFileProcessorProcess, with no API client. A failed start, a missing or invalid result, or a timeout is an import error keyed by the artifact's path in its bundle.
• Requests from the runtime that need a client are relayed up the Dag-parsing child's own supervisor channel.
• The runtime's channel accepts only ToManager, where a DagFileParsingResult is an unhandled request, so a Dag registration is never read as a task handler.
• The runtime dies with the Dag-parsing child: it runs with PR_SET_PDEATHSIG on Linux, and what it leaves in its process group is killed on exit or timeout.
• The methods shared with Parse native Lang-SDK Dags in the Dag processor with their runtime #74035's class keep its names and bodies. Lang-SDK runtimes: one transport for native Dag parsing and the task-handler probe #74058 moves both classes onto one transport.
• A child of a supervised child no longer closes its own new stdin, stdout and stderr when it replaces the handles it inherited. This affects every supervised child, and the probe runs in one.
• ADR-0012 and ADR-0013 describe the probe as built.

Known limits

• Where the Dag processor starts children with exec instead of fork (the macOS default), the probe inherits the Dag-parsing child's ORM-blocking environment and dies at import airflow. The check then logs a warning and leaves the stub tasks unchecked. Linux is not affected.
• The parent death signal reaches only the runtime itself. If the Dag-parsing child is killed abruptly, processes the runtime started survive, so a runtime must exec and leave no children (Lang-SDK task handlers: smaller follow-ups #74059).

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