fix(optimizer): unblock server.close() when deps were discovered before listen (fix #23143)

[santusht06]

Description

Fixes #23143.

When server.transformRequest(...) encounters a bare specifier without server.listen() being called (e.g. in test suites using Vite's programmatic JavaScript API):

1. A dependency record is created with info.processing set to depOptimizationProcessing.promise.
2. The import is rewritten to /.vite/deps/..., and when loaded, vite:optimized-deps awaits info.processing.
3. Because server.listen() was never called (deferred since fix(optimizer): avoid optimizer run for transform request before init #22852), depsOptimizer.init() never runs and the pre-bundling run never starts.
4. EnvironmentPluginContainer registers this hook promise in this._processesing.
5. On server.close(), pluginContainer.close() waits on Promise.allSettled(this._processesing) which blocks indefinitely on the unfulfilled info.processing promise.

Solution

• Resolve depOptimizationProcessing and queued promises (resolveEnqueuedProcessingPromises()) inside depsOptimizer.close().
• Clear pending debounce and logging timer handles (debounceProcessingHandle, newDepsToLogHandle) on close.
• Add isClosed: () => boolean on DepsOptimizer and gracefully return null in optimizedDeps.load() when reading unbundled files during/after server shutdown.
• Add regression test in discoverBeforeListen.spec.ts.

Validation

• pnpm run build
• pnpm vitest run packages/vite/src/node/__tests__/optimizer/discoverBeforeListen.spec.ts
• pnpm run typecheck
• pnpm run lint
• Verified against reproduction repository (resolved 10s timeout failure to 1ms exit).