Show "tab Accept" only for zeta (#23463)

#23460 brought up we are showing the new "tab Accept" marker for single
line suggestions for non-zeta providers. We think this might be valid
for any provider, but we only want to enable it for zeta initially so it
doesn't affect an existing user base.

Release Notes:

- N/A

crates/inline_completion/src/inline_completion.rs

@@ -22,6 +22,9 @@ pub trait InlineCompletionProvider: 'static + Sized {
     fn display_name() -> &'static str;
     fn show_completions_in_menu() -> bool;
     fn show_completions_in_normal_mode() -> bool;
+    fn show_tab_accept_marker() -> bool {
+        false
+    }
     fn is_enabled(
         &self,
         buffer: &Model<Buffer>,
@@ -67,6 +70,7 @@ pub trait InlineCompletionProviderHandle {
     ) -> bool;
     fn show_completions_in_menu(&self) -> bool;
     fn show_completions_in_normal_mode(&self) -> bool;
+    fn show_tab_accept_marker(&self) -> bool;
     fn needs_terms_acceptance(&self, cx: &AppContext) -> bool;
     fn is_refreshing(&self, cx: &AppContext) -> bool;
     fn refresh(
@@ -113,6 +117,10 @@ where
         T::show_completions_in_normal_mode()
     }
 
+    fn show_tab_accept_marker(&self) -> bool {
+        T::show_tab_accept_marker()
+    }
+
     fn is_enabled(
         &self,
         buffer: &Model<Buffer>,