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

assets/settings/default.json

@@ -160,6 +160,9 @@
   /// Whether to show the signature help after completion or a bracket pair inserted.
   /// If `auto_signature_help` is enabled, this setting will be treated as enabled also.
   "show_signature_help_after_edits": false,
+  /// Whether to show the inline completions next to the completions provided by a language server.
+  /// Only has an effect if inline completion provider supports it.
+  "show_inline_completions_in_menu": true,
   // Whether to show wrap guides (vertical rulers) in the editor.
   // Setting this to true will show a guide at the 'preferred_line_length' value
   // if 'soft_wrap' is set to 'preferred_line_length', and will show any