fix: allow symlinked dev server roots

[popsiclelmlm]

Summary

• include realpath counterparts for server.fs.allow directories so symlinked project roots can load files resolved through their target path
• add coverage for symlinked root module loading and explicit fs.allow symlink entries

Fixes #16257

Tests

• pnpm --filter vite build-bundle
• pnpm exec vitest run packages/vite/src/node/server/tests/server-options.spec.ts
• pnpm --filter vite exec oxfmt --check src/node/server/index.ts src/node/server/tests/server-options.spec.ts
• pnpm --filter vite exec eslint src/node/server/index.ts src/node/server/tests/server-options.spec.ts
• pnpm --filter vite build-types
• pnpm --filter vite typecheck
• git diff --check