fix: update hmr import paths to include .js extension

[chenjiahan]

Summary

Update the hmr import path to include .js extension as @rsbuild/core package includes the type: "module" field.

Checklist

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

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[Copilot]

Pull Request Overview

This PR updates import paths in the client code to include the .js extension for ES module compatibility, as required by the "type": "module" field in the package.json.

• Updated import statement in overlay.ts to reference './hmr.js' instead of './hmr'
• Updated externals configuration in rslib.config.ts to match the new import path

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/core/src/client/overlay.ts	Updated import path to include .js extension for ES module compatibility
packages/core/rslib.config.ts	Updated externals configuration to match the corrected import path with .js extension

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