Add Vim undo to the TUI composer (#41941)

## Why

Composer drafts include attachments, mention targets, and deferred paste payloads in addition to visible text. Vim undo needs to restore that complete state as a single edit.

## What changed

- Add bounded, draft-level Vim undo history, with `u` as the configurable default binding in normal mode.
- Group complete Vim commands and insert sessions into undo steps, including direct composer changes such as pastes and attachments.
- Preserve undo, repeat, and search state when a reverse-history preview is canceled, and start fresh history when another prompt is accepted.

## Testing

- Cover grouped edits, custom bindings, history and search interactions, attachments, and large paste payloads.

GitOrigin-RevId: 12ecdb3c34a2d66f5d856aa2f82dc15bd91b230b

codex-rs/core/config.schema.json

@@ -3902,6 +3902,7 @@
               "substitute_char": null,
               "till_backward": null,
               "till_forward": null,
+              "undo": null,
               "yank_line": null
             },
             "vim_operator": {
@@ -4728,6 +4729,7 @@
             "substitute_char": null,
             "till_backward": null,
             "till_forward": null,
+            "undo": null,
             "yank_line": null
           }
         },
@@ -5246,6 +5248,14 @@
           ],
           "description": "Stop before the next character on the current line (`t`)."
         },
+        "undo": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/KeybindingsSpec"
+            }
+          ],
+          "description": "Undo the last complete edit (`u`)."
+        },
         "yank_line": {
           "allOf": [
             {