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/zeta/src/zeta.rs

@@ -932,6 +932,10 @@ impl inline_completion::InlineCompletionProvider for ZetaInlineCompletionProvide
         "Zeta"
     }
 
+    fn show_completions_in_menu() -> bool {
+        true
+    }
+
     fn is_enabled(
         &self,
         buffer: &Model<Buffer>,