Hi all – we will have a permanent fix for this in the next update to the Cursor client.
In the meantime, could you install the original ionide.ionide-fsharp extension, and then monkeypatch its package.json to move the C# dependency to the “extensionPack” section from the “extensionDependencies”? See After installing the 'C/C++' extension from Anysphere, 'PlatformIO IDE' is not activated - #14 by JoergSH as a guide.