chat: toggle queue/steer keybindings based on context (#299885)

* chat: toggle queue/steer keybindings based on context

- Splits EditingRequestType.QueueOrSteer into Queue and Steer to track
  which type of pending message is being edited
- Keybindings now respect the chat.requestQueuing.defaultAction setting:
  when steer is default, Enter=Steer and Alt+Enter=Queue; when queue is
  default, the bindings swap
- When editing a queued or steer message, Enter always submits with the
  same type, regardless of the config setting. This ensures pressing Enter
  to save an edit keeps the message in its original queue category
- Updates chatWidget to set the specific editing type based on the
  pending message's kind
- Simplifies keybinding logic with effectiveDefault conditions that
  account for both config and editing context

Fixes #297454

(Commit message generated by Copilot)

* pr comments