acp: Support session modes (e.g. CC plan mode) (#37632)

Adds support for [ACP session
modes](https://github.com/zed-industries/agent-client-protocol/pull/67)
enabling plan and other permission modes in CC:


https://github.com/user-attachments/assets/dea18d82-4da6-465e-983b-02b77c6dcf15


Release Notes:

- Claude Code: Add support for plan mode, and all other permission modes

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

assets/keymaps/default-macos.json

@@ -378,6 +378,12 @@
       "ctrl--": "pane::GoBack"
     }
   },
+  {
+    "context": "AcpThread > ModeSelector",
+    "bindings": {
+      "cmd-enter": "menu::Confirm"
+    }
+  },
   {
     "context": "AcpThread > Editor && !use_modifier_to_send",
     "use_key_equivalents": true,
@@ -385,7 +391,8 @@
       "enter": "agent::Chat",
       "shift-ctrl-r": "agent::OpenAgentDiff",
       "cmd-shift-y": "agent::KeepAll",
-      "cmd-shift-n": "agent::RejectAll"
+      "cmd-shift-n": "agent::RejectAll",
+      "shift-tab": "agent::CycleModeSelector"
     }
   },
   {
@@ -395,7 +402,8 @@
       "cmd-enter": "agent::Chat",
       "shift-ctrl-r": "agent::OpenAgentDiff",
       "cmd-shift-y": "agent::KeepAll",
-      "cmd-shift-n": "agent::RejectAll"
+      "cmd-shift-n": "agent::RejectAll",
+      "shift-tab": "agent::CycleModeSelector"
     }
   },
   {