agent: Fix message editor expand binding (#30553)

As of https://github.com/zed-industries/zed/pull/30504, we now can zoom
in the whole panel, which uses the `shift-escape` keybinding. We were
also using the same binding for the message editor expansion, which was
caused a conflict. Now, the message editor expansion requires an
additional key (`alt`) to work.

Release Notes:

- agent: Fixed conflicting keybinding between message editor and panel
zoom.

assets/keymaps/default-linux.json

@@ -244,7 +244,7 @@
       "ctrl-shift-a": "agent::ToggleContextPicker",
       "ctrl-shift-o": "agent::ToggleNavigationMenu",
       "ctrl-shift-i": "agent::ToggleOptionsMenu",
-      "shift-escape": "agent::ExpandMessageEditor",
+      "shift-alt-escape": "agent::ExpandMessageEditor",
       "ctrl-alt-e": "agent::RemoveAllContext",
       "ctrl-shift-e": "project_panel::ToggleFocus"
     }

assets/keymaps/default-macos.json

@@ -290,7 +290,7 @@
       "cmd-shift-a": "agent::ToggleContextPicker",
       "cmd-shift-o": "agent::ToggleNavigationMenu",
       "cmd-shift-i": "agent::ToggleOptionsMenu",
-      "shift-escape": "agent::ExpandMessageEditor",
+      "shift-alt-escape": "agent::ExpandMessageEditor",
       "cmd-alt-e": "agent::RemoveAllContext",
       "cmd-shift-e": "project_panel::ToggleFocus"
     }