fix(type): make the PostCSS plugin type loose

[chenjiahan]

Summary

When user passes a PostCSS plugin to tools.postcss config, the plugin may depend on a different version of PostCSS, which can cause type errors. This PR loosens the PostCSS plugin type to resolve type issue.

Related Links

Fix ecosystem CI: https://github.com/rspack-contrib/rsbuild-ecosystem-ci/actions/runs/15458968100/job/43516378827

Checklist

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

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[Copilot]

Pull Request Overview

This PR loosens the PostCSS plugin type to avoid type errors when users supply plugins built against different PostCSS versions.

• Changed PostCSSOptions.plugins to accept unknown[] instead of a strict plugin type
• Updated ToolsPostCSSLoaderConfig.addPlugins to use unknown types
• Adapted CSS plugin loader to store and map plugins as unknown, adding runtime casts

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
packages/core/src/types/thirdParty.ts	Made plugins property accept unknown[]
packages/core/src/types/config.ts	Loosened addPlugins signature to unknown types
packages/core/src/plugins/css.ts	Updated extraPlugins array and plugin-mapping casts