Fix patch webview asset resolution in source checkouts

[Copilot]

Email-style patch files could open with a blank visual view even though the patch content itself was valid. The issue was not diff parsing; the webview could fail to load diff2html assets when running from a fresh clone or local source checkout without copied media assets.

• Runtime asset resolution

  • Resolve diff2html assets from media/diff2html when packaged assets are present.
  • Fall back to node_modules/diff2html/bundles for local development and test runs.
  • Emit a clearer log entry when neither asset location is available.

• Focused regression coverage

  • Add tests for both resolution paths:
    • packaged media assets win when present
    • node_modules is used when media assets are absent
  • Wrap temp-directory setup in try/finally to keep test cleanup deterministic.

• Changelog

  • Note the local asset-loading fix under Unreleased.

const mediaAssetRoot = path.join(extensionPath, 'media', 'diff2html');
if (hasDiff2HtmlAssets(mediaAssetRoot)) {
    return mediaAssetRoot;
}

const nodeModulesAssetRoot = path.join(extensionPath, 'node_modules', 'diff2html', 'bundles');
if (hasDiff2HtmlAssets(nodeModulesAssetRoot)) {
    return nodeModulesAssetRoot;
}