fix(ext/node): keep worker alive while holding a refed MessagePort

[divybot]

Summary

A node:worker_threads Worker that holds a "refed" transferable object such as a MessagePort must not be terminated on idle. This covers the use cases from #23169:

• a worker pool using a dedicated MessagePort for IPC, with a minimum number of workers always kept alive, and
• a worker that is set up, idle, and waiting for work.

Root cause

The runtime idle-termination check hasMessageEventListener() in runtime/js/99_main.js consults messagePort.refedMessagePortsCount to decide whether a node worker should stay alive:

return (event.listenerCount(globalThis, "message") > 0 &&
  !globalThis[messagePort.unrefParentPort]) ||
  messagePort.refedMessagePortsCount > 0;

That counter is a mutable module-level let in ext/web/13_message_port.js. When ext/web was converted to lazy-loaded IIFE scripts in #33760, the module's real ESM export { refedMessagePortsCount } (a live binding) became a plain property on the returned object literal, capturing a one-time snapshot of 0. The refed-port branch of the idle check was therefore dead, and a worker kept alive only by a refed MessagePort (with no active "message" listener op promise) was incorrectly terminated on idle.

Fix

Restore the live-binding semantics by exposing refedMessagePortsCount as a getter on the module's return object.

Tests

Adds two spec tests under tests/specs/node/worker_threads/:

• refed_messageport_idle — a worker receives delayed work over a refed transferred MessagePort after an idle period.
• refed_messageport_holder — a worker kept alive purely by a refed MessagePort it does not listen on. This isolates the fix: it fails on main (worker terminates on idle) and passes with the change.

Closes #23169

Closes denoland/divybot#429