fix(build): improve root URL import error

[Caltsic]

Fixes #10871.

Summary

• Add a targeted hint when build fails on a root URL import (/).
• Cover the case with a focused build unit test.

Details

Vue template asset references such as <img src=/> are transformed into an import of /. The resolver now reports this as an unresolved import with a code frame, but the message does not explain the asset reference problem. This keeps resolution behavior unchanged and augments the build error only for root URL imports.

Tests

• pnpm vitest run packages/vite/src/node/__tests__/build.spec.ts
• pnpm exec oxfmt --check packages/vite/src/node/build.ts packages/vite/src/node/__tests__/build.spec.ts
• pnpm exec eslint --cache --max-warnings 0 packages/vite/src/node/build.ts packages/vite/src/node/__tests__/build.spec.ts
• pnpm --filter vite typecheck
• pnpm --filter vite build-bundle