Revert "fix(type): make the PostCSS plugin type loose (#5364)"

[chenjiahan]

This reverts commit 8772871.

Summary

Revert #5364

This change does not fix the ecosystem CI. And considering that PostCSS types do not change frequently, it may be a better approach to let users upgrade the PostCSS version.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR reverts the previous loose typing for PostCSS plugins and restores stricter AcceptedPlugin/PostCSSPlugin types throughout the core and plugin code.

• Changes PostCSSOptions.plugins from unknown[] to AcceptedPlugin[]
• Narrows the addPlugins callback in ToolsPostCSSLoaderConfig to PostCSSPlugin
• Updates getPostcssLoaderOptions to use AcceptedPlugin[] for extraPlugins

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/core/src/types/thirdParty.ts	Restored plugins property to use AcceptedPlugin[]
packages/core/src/types/config.ts	Updated addPlugins signature to accept PostCSSPlugin
packages/core/src/plugins/css.ts	Changed extraPlugins and related logic to AcceptedPlugin

Comments suppressed due to low confidence (3)

packages/core/src/types/thirdParty.ts:22

• Restricting the plugins property from unknown[] to AcceptedPlugin[] is a breaking change for existing consumers. Consider bumping the major version or documenting this change in the migration guide.

plugins?: AcceptedPlugin[];

packages/core/src/types/config.ts:64

• Narrowing addPlugins to PostCSSPlugin may break existing callback usage. Make sure to update public API docs and communicate this change in release notes.

{ addPlugins: (plugins: PostCSSPlugin | PostCSSPlugin[]) => void }

packages/core/src/plugins/css.ts:147

• This stricter typing for extraPlugins warrants adding tests to cover both PluginCreator and raw plugin scenarios via addPlugins to ensure compatibility.

const extraPlugins: AcceptedPlugin[] = [];