test: css addWatchFile in load hook

[bluwy]

Description

Adds a failing test for CSS that relies on the load hook and calls this.addWatchFile there, which doesn't work currently (in v6 only).

The reason is because of here:

vite/packages/vite/src/node/server/pluginContainer.ts

Lines 196 to 204 in ba56cf4

	private _updateModuleLoadAddedImports(
	id: string,
	addedImports: Set<string> | null,
	): void {
	const module = this.moduleGraph?.getModuleById(id)
	if (module) {
	this._moduleNodeToLoadAddedImports.set(module, addedImports)
	}
	}

The module does not exist until the load hook finishes here:

vite/packages/vite/src/node/server/transformRequest.ts

Lines 338 to 340 in ba56cf4

	// ensure module in graph after successful load
	mod ??= await moduleGraph._ensureEntryFromUrl(url, undefined, resolved)

I'm not entirely sure why it worked in v5 yet (for SPA only). But for SSR, I know that since the module nodes are shared, the pluginContainer can share the client module node after ssr already created the module node.

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