Fix Open VSX marketplace URL gallery path issue

[jinvien]

Description
This PR fixes a URL path issue in the Open VSX marketplace integration that was causing incorrect redirects.

Problem
vscode extension gallery was generating incorrect URLs with an extra /vscode segment:

❌ Incorrect: https://open-vsx.org/vscode/gallery/vscode/eamodio/gitlens/latest
✅ Correct: https://open-vsx.org/vscode/gallery/eamodio/gitlens/latest
The incorrect URL format was causing redirects to http://open-vsx.org/error instead of properly resolving extension resources.

Changes
Fixed the extension latest version URI template in extensionGalleryManifestService.ts by removing the redundant /vscode segment

[code-asher]

Ohhhhhhh interesting is this why extension installations sometimes fail??

But, I am not sure if we can merge this because there are other marketplaces (such as code-marketplace) that do include the /vscode so this would break those.

Also, what about other VS Code-based alternatives like VSCodium? I imagine they must have the same bug. Maybe this needs to be fixed on Open VSX's side?

If not, then maybe we will need to pull this off extensionsGallery instead so it can be customized.

[code-asher]

Actually, eclipse/openvsx#1283 suggests we can set extensionUrlTemplate on extensionsGallery to fix this. Looks like VSCodium works because it sets this, so we can do the same (in this same patch).

[jinvien]

Yes, this is clearly a more elegant solution. I've updated the commit accordingly—please take a look when you have a moment.

Also, I’m not entirely sure whether my commit should include changes to changelog.md. I noticed in your contribution guidelines that updates related to the marketplace should be reflected in the changelog. Could you please advise on whether that’s needed here?

Thanks!