fix(cli): hint preview for missing default entry

[Sylar-W]

Summary

• Warn when rspack serve runs without a config and the default ./src entry is missing.
• Point users who are trying to serve build output toward rspack preview [dir].
• Add a regression test for the missing default entry case.

Related links

Refs #13799

Test Plan

• NAPI_RS_NATIVE_LIBRARY_PATH="$PWD/node_modules/.pnpm/@rspack+binding-darwin-arm64@2.0.0/node_modules/@rspack/binding-darwin-arm64/rspack.darwin-arm64.node" pnpm --filter @rspack/cli test -- tests/serve/missing-default-entry/index.test.ts
• pnpm exec prettier --check packages/rspack-cli/src/cli.ts packages/rspack-cli/tests/serve/missing-default-entry/index.test.ts
• pnpm exec rslint packages/rspack-cli/src/cli.ts
• git diff --check

Checklist

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

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Detect resolvable default entry instead of filesystem presence

The new hasDefaultEntry helper only checks whether ./src, ./src.js, or ./src.json exists on disk, but rspack still fails when ./src exists as a directory without a resolvable default module (for example, an empty src/ folder or a src/ folder containing only main.ts). In those cases rspack serve still errors on resolving ./src, yet this warning is suppressed, so the commit’s new guidance is skipped for a real missing-default-entry case.

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