Make completions menu stay open after after it's manually requested (#32015)

Also includes a clarity refactoring to remove
`ignore_completion_provider`.

Closes #15549

Release Notes:

- Fixed completions menu closing on typing after being requested while
`show_completions_on_input: false`.

crates/collab_ui/src/chat_panel/message_editor.rs

@@ -89,6 +89,7 @@ impl CompletionProvider for MessageEditorCompletionProvider {
         _position: language::Anchor,
         text: &str,
         _trigger_in_words: bool,
+        _menu_is_open: bool,
         _cx: &mut Context<Editor>,
     ) -> bool {
         text == "@"