fix(unplugin): prevent file handle leaks in Vitest (#1533)

[lecstor]

What changed / motivation ?

This PR fixes two resource-leak issues in @stylexjs/unplugin that cause
Vitest's hanging-process reporter to detect ~581 leaked FILEHANDLE
operations and 1 leaked Timeout, preventing clean process exit.

Linked PR/Issues

Fixes #1533

Additional Context

Root Cause

1. Babel config-file discovery — FILEHANDLE ×581

runBabelTransform() passes babelrc: false but does not set configFile.
When configFile is not explicitly set to false, Babel proceeds with root
config file discovery, triggering up to 7 parallel file-system probes for config
file variants (babel.config.{js,cjs,mjs,json,cts,mts,ts}) on every
transformAsync() invocation. In async mode, these use callback-based
fs.readFile/fs.stat operations whose handles are tracked as async resources
by Vitest's reporter. In a project with ~80 StyleX modules, this produces ~581
leaked handle operations.

2. Unreferenced polling timer — Timeout ×1

The configureServer() hook starts a setInterval(..., 150) to poll the shared
CSS-rules version counter. It attaches a cleanup listener to
server.httpServer?.once('close', ...), but in Vitest's middleware mode
httpServer is null. The interval therefore runs forever, keeping the Node.js
event loop alive.

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code
• Both changes are minimal, targeted, one-line fixes
• babelrc: false was already set — adding configFile: false is the
  symmetric completion of "don't look for external config"
• interval.unref() is the standard Node.js pattern for optional timers
  (used in Vite core itself)
• No new dependencies added
• No changes to public API or user-facing behavior
• The server.httpServer?.once('close', ...) cleanup listener is preserved
  as a belt-and-suspenders safeguard for non-middleware environments
• Tested with vitest run --reporter=hanging-process — zero leaked
  resources

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