fix(asset): prevent removal of programmatically created assets

[DipakHalkude]

What does it do?

Fixes issue #5680 where programmatically created assets in processors were being removed by the asset generator.

The asset generator was checking for physical file existence and removing all assets without corresponding files, including those created programmatically with modified: true flag.

Solution

Modified the asset generator to skip removal for assets marked with modified: true. This allows plugins and processors to create virtual/assets programmatically that persist in the database.

Changes Made

1. lib/plugins/generator/asset.ts: Added check for asset.modified flag before removing assets
2. test/scripts/generators/asset.test.js: Added comprehensive tests

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

Testing

• ✅ All tests pass
• ✅ TypeScript build succeeds
• ✅ Backward compatible - non-modified assets still get cleaned up normally
• ✅ Allows programmatic asset creation as requested in Save database such as Asset in processor is not working #5680

Example Usage

hexo.extend.processor.register("_posts/:slug\.md", function(file) {
  const Asset = this.model('Asset');
  const test = Asset.findById("Test");
  if (!test) {
    return Asset.insert({
      _id: "Test",
      path: "Test",
      modified: true  // This prevents removal
    });
  }
});

[github-actions]

How to test

git clone -b fix-asset-generator-removal https://github.com/DipakHalkude/hexo.git
cd hexo
npm install
npm test

[DipakHalkude]

@SukkaW @curbengh @D-Sketon Could you please review this fix for #5680? It addresses the issue where programmatic assets weren't persisting in the database.