TS SDK: answer TaskHandlerParseRequest from bundle.serve

[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

bundle.serve() now answers #73974's probe with every TaskHandler the bundle registers, and NodeCoordinator can find and start the bundle a stub task would run. Until now a TypeScript bundle failed on a TaskHandlerParseRequest as an unsupported first frame. Nothing calls the probe yet: #74135 does.

bundle.register(
  new TaskHandler("etl", "summarize", summarize),
  new TaskHandler("etl", "report", withArgNames({ label: "run_label" }, report)),
);
await bundle.serve();

answers:

TaskHandlerParsingResult(
    fileloc="/bundles/ts/etl.min.mjs",
    task_handlers={"etl": [
        TaskHandlerDeclaration(task_id="summarize", binding="named", params=None),
        TaskHandlerDeclaration(task_id="report", binding="named", params=None),
    ]},
)

• Only TaskHandlers are declared, keyed by Dag id in registration order. Tasks of a Dag declared in TypeScript are left out.
• TypeScript types are erased, so the SDK cannot list a handler's params, even with withArgNames. Each handler is named with params=None, and the Dag processor checks only that it exists.
• bundle.serve() waits up to 30 s for the parent to acknowledge the answer. An error, a closed socket or a timeout fails the process.
• The probe checks the bundle a worker would run: _find_task_handler_artifact is the worker's own scan, and _build_parse_task_handler_command is native Parse native TypeScript Dags with the Node coordinator #74037's _build_bundle_command, which verifies the bundle and runs it with node as a task does.

Compatibility: a bundle packed from main before this change reports 2026-10-30 but cannot answer, so its probe fails and #74135 logs a warning. Packing it again fixes it. A bundle packed with the published 0.1.0-beta1 is a bundle.mjs, which the scan does not find, so it is not probed (a worker cannot run it either).

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