Show inline completions in menu only for supported providers (#22181)

This changes #22093 by making the change only have an effect for the
supported provider: Zeta.

Made the change because the UX is still experimental and I don't want to
break existing workflows for Copilot/Supermaven users.

Even Zeta users can opt-out of it by setting
`"show_inline_completions_in_menu": false` in their settings, in case
they want the old show-inline-completion-or-show-lsp-completion behavior
back.

Release Notes:

- N/A

crates/supermaven/src/supermaven_completion_provider.rs

@@ -102,6 +102,10 @@ impl InlineCompletionProvider for SupermavenCompletionProvider {
         "Supermaven"
     }
 
+    fn show_completions_in_menu() -> bool {
+        false
+    }
+
     fn is_enabled(&self, buffer: &Model<Buffer>, cursor_position: Anchor, cx: &AppContext) -> bool {
         if !self.supermaven.read(cx).is_enabled() {
             return false;