Go's test generation and interface stubs were reachable only through context actions, because Generate filtered their titles, and implementing an interface on a plain struct wasn't available since the client didn't support gopls's interactive commands.
Generate and the editor's context menu now list `Add test for ...`, `Declare missing methods ...` and `Add methods to ... implement ...`, and a generated test file opens in the editor. For gopls only, the client advertises `interactiveResolve` and runs its `command/resolve` handshake: a prompt titled from the server asks for the interface, shows validation errors in place, and Cancel writes nothing. Requests are cancellable and other servers and existing commands are unchanged.
Fixes#620