fix(html): fix CSS missing when building with different seenCss context

[thy486]

Previously, CSS file cache was built based on seenCss state at that time. When multiple HTML entries process the same chunk, the cached CSS files may exclude files that were already in seenCss during first processing, causing those files to be missing in subsequent entries.

This fix introduces dirty chunk tracking to validate and rebuild cache when CSS files are detected as missing.

e.g.

  const bundle4: MockBundle = {
    'entry.js': {
      fileName: 'entry.js',
      type: 'chunk',
      imports: ['chunk1.js', 'chunk2.js'],
      viteMetadata: { importedCss: ['entry.css'] },
    },
    'entry2.js': {
      fileName: 'entry2.js',
      type: 'chunk',
      imports: ['chunk2.js'],
      viteMetadata: { importedCss: ['entry2.css'] },
    },
    'chunk1.js': {
      fileName: 'chunk1.js',
      type: 'chunk',
      imports: [],
      viteMetadata: { importedCss: ['chunk1.css', 'chunk-shared.css'] },
    },
    'chunk2.js': {
      fileName: 'chunk2.js',
      type: 'chunk',
      imports: [],
      viteMetadata: { importedCss: ['chunk2.css', 'chunk-shared.css'] },
    },
  }

  const oldGetCss4 = createGetCssFilesForChunkOld(bundle4)
  oldGetCss4(bundle4['entry.js']) // Process the first entry and set up the cache.
  // ✅ Output: chunk-shared.css, chunk1.css, chunk2.css, entry.css
  const oldResult4 = oldGetCss4(bundle4['entry2.js'])
  // ❌ Output: chunk2.css, entry2.css
  const newGetCss4 = createGetCssFilesForChunkNew(bundle4)
  newGetCss4(bundle4['entry.js'])
  // ✅ Output: chunk-shared.css, chunk1.css, chunk2.css, entry.css
  const newResult4 = newGetCss4(bundle4['entry2.js'])
  // ✅ Output: chunk-shared.css, chunk2.css, entry2.css

Tips: This code is not ready for immediate use. I'm just presenting my modification proposal first.

[sapphi-red]

Would you extract this part of code and write unit tests? It would be great if you can do the extraction and add unit tests for #19143 in a separate PR first.