/plugins: remove marketplace (#19843)

This PR adds marketplace removal to the /plugins menu, giving users a
way to remove user-configured plugin marketplaces. It adds a `Ctrl+R`
shortcut to remove selected marketplace tabs, a confirmation prompt,
loading and error states, and the app-server request flow needed to
perform marketplace/remove. After a successful removal, the TUI
refreshes config, plugin mentions, user config, and plugin data so the
removed marketplace disappears from the menu and other surfaces in the
TUI.

- Add `Ctrl+R` removal option for user-configured marketplace tabs
- Show marketplace removal confirmation, loading, and error states
- Route `marketplace/remove` through the TUI background request flow
- Refresh config, plugin mentions, and plugin data after successful
removal
- Adds reusable per-tab footer hints so removal guidance only appears on
applicable tabs
- Add test coverage for `Ctrl+R` behavior while plugin search is active

Steps to test:
- Add a marketplace using the TUI /plugins menu
- Use Ctrl+R to remove the marketplace
- Accept the confirmation prompt
- Confirm the marketplace is removed when the process completes.

codex-rs/tui/src/chatwidget.rs

@@ -5486,6 +5486,7 @@ impl ChatWidget {
                     ..
                 } if modifiers.contains(KeyModifiers::CONTROL) && c.eq_ignore_ascii_case(&'c')
             )
+            && !key_hint::ctrl(KeyCode::Char('r')).is_press(key_event)
         {
             self.bottom_pane.handle_key_event(key_event);
             if self.bottom_pane.no_modal_or_popup_active() {
@@ -5603,6 +5604,10 @@ impl ChatWidget {
             return;
         }
 
+        if self.handle_plugins_popup_key_event(key_event) {
+            return;
+        }
+
         match key_event {
             KeyEvent {
                 code: KeyCode::BackTab,