refactor(computer-use): injectable win32 runner for host-agnostic backend tests

[EvanProgramming]

Summary

Refactor-only follow-up, as invited on the closed #5910: the win32 input bug was already fixed and merged in #5903 — this PR does not re-touch that logic. It keeps main's merged behavior and adds the instance-local injectable runner seam that was called out as cleaner than the fake-powershell.exe-on-PATH fixture.

Why

crates/tui/plugins/computer-use/src/backends/win32.mjs defined its PowerShell runner helpers (ps/psOk/psJson) at module scope, so backend tests either needed a real powershell.exe or a PATH fixture. Moving them into create() lets a test pass create({ exec: { run } }) — the exact injection pattern the harmonyos backend already uses.

Changes

• create(opts = {}): ps/psOk/psJson become instance-local and default to the production runner; opts.exec.run overrides it. No production behavior change when create() is called with no args.
• New tests/win32.test.mjs exercising the win32 backend on any host (no Windows, no fake powershell):
  • actions run through an injected runner;
  • a nonzero exit rejects the action (fail-truthful behavior from fix(computer-use): win32 backend reports PowerShell failures truthfully #5903 is pinned);
  • targeted left_mouse_down moves and presses, with the User32 type defined in-process;
  • every User32 action carries the type prelude.

Verification

node --test tests/win32.test.mjs → 4/4 pass; node --test tests/backends.test.mjs → 10/10 pass (no regression). All on macOS.

Refs #5903, #5910.

[github-actions]

Thanks @EvanProgramming for taking the time to contribute.

This repository is observing a maintainer-managed PR intake gate in dry-run mode, so this pull request is staying open. This note helps maintainers prepare the allowlist before any enforcement is considered.

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[Hmbown]

Verified locally on the PR head: the plugin suite passes (47/47 via npm test), the create({ exec }) seam matches the harmonyos backend's shape, and create() with no args is behavior-identical to main. The four new tests pin exactly the #5903 guarantees (in-process User32 prelude, nonzero exit rejects, targeted left_mouse_down moves and presses) without a Windows host. Merging once CI finishes.