assistant: Add model selector to the Context Editor (#25032)

This PR also removes everything related with the model selector from the
Context Editor toolbar.

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -587,6 +587,7 @@
       "save": "workspace::Save",
       "ctrl->": "assistant::QuoteSelection",
       "ctrl-<": "assistant::InsertIntoEditor",
+      "ctrl-alt-/": "assistant::ToggleModelSelector",
       "shift-enter": "assistant::Split",
       "ctrl-r": "assistant::CycleMessageRole",
       "enter": "assistant::ConfirmCommand",