fix(app-tools): avoid resolving ts paths from non-file parents

[GiveMe-A-Name]

Summary

• avoid running the ts paths loader against non-file: parent URLs, which can be introduced by PostCSS/rsbuild virtual modules when loading postcss.config.mjs
• keep runtime alias resolution scoped to app files so the loader only participates where tsconfig path aliases are expected to apply
• add regression coverage for the Tailwind v4 + postcss.config.mjs + tsconfig.json crash and preserve the alias runtime boundary checks

Validation

• pnpm --filter @modern-js/app-tools build
• pnpm --dir tests exec rstest integration/tailwindcss/tests/tailwindcss-v4.test.ts
• pnpm --dir tests exec rstest integration/alias-set/tests/index.test.ts
• verified the external repro from issue [Bug]: Error: Loading PostCSS Plugin failed: The URL must be of scheme file #8516 no longer crashes on pnpm dev after consuming the patched @modern-js/app-tools

Closes #8516

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