debugger: Add actions and keybindings for opening the thread and session menus (#31135)

Makes it possible to open and navigate these menus from the keyboard.

I also removed the eager previewing behavior for the thread picker,
which was buggy and came with a jarring layout shift.

Release Notes:

- Debugger Beta: Added the `debugger: open thread picker` and `debugger:
open session picker` actions.

crates/debugger_ui/src/debugger_ui.rs

@@ -45,6 +45,8 @@ actions!(
         FocusLoadedSources,
         FocusTerminal,
         ShowStackTrace,
+        ToggleThreadPicker,
+        ToggleSessionPicker,
     ]
 );