fix(module-runner): initialize CSS module cache for non-client enviro…

[rexxars]

Description

Importing CSS files via createServerModuleRunner in dev mode throws a TypeError because the CSS module cache is never initialized for non-client environments.

• .module.css: TypeError: Cannot read properties of undefined (reading 'set')
• Plain .css: TypeError: Cannot read properties of undefined (reading 'get')

Root cause

Both cssPlugin and cssPostPlugin depend on a moduleCache / cssModulesCache that is only initialized inside the buildStart hook. However, in the dev server's plugin container, buildStart is conditionally called per-plugin - it only fires for plugins that have perEnvironmentStartEndDuringDev: true or when the environment is "client". Since the CSS plugins don't set that flag, their buildStart callback is skipped for the SSR environment, leaving the cache undefined.

When using middlewareMode: true, this is masked because initServer() explicitly calls buildStart on the client environment, and the shared plugin closure variable gets initialized. But without middleware mode (the common
createServerModuleRunner pattern), no buildStart fires before the first CSS transform, causing the crash.

You can work around this by running await server.pluginContainer.buildStart({}) first, but it seems a little hacky/non-intuitive.

Fix

• Add a configureServer hook to cssPlugin that eagerly initializes moduleCache and cssModulesCache if not already set. This hook always runs during createServer(), regardless of middleware mode or environment.
• Change the unsafe non-null assertion (!) in cssPostPlugin to optional chaining (?.), consistent with how cssAnalysisPlugin already accesses the same cache.

Testing

Added a test that creates a server without middlewareMode and imports both .css and .module.css files through createServerModuleRunner. This test fails before the fix and passes after.