Add Vim replace mode to the TUI composer (#42194)

## What changed

- Add Vim Replace mode, entered with `R`, which overwrites graphemes and appends at the end of a line.
- Restore overwritten text with Backspace and record replace edits for undo and dot-repeat.
- Preserve composer behavior for attachments, completions, paste bursts, history previews, and external editor imports while replacing text.
- Expose `vim_normal.enter_replace_mode` in the configurable TUI keymap.

## Testing

- Add coverage for grapheme replacement, Backspace recovery, dot-repeat, custom bindings, atomic attachments, completions, paste handling, and history interactions.

GitOrigin-RevId: cb04e3adcce013c1eb7b82c994b00e944310c481

codex-rs/core/config.schema.json

@@ -3927,6 +3927,7 @@
               "delete_char": null,
               "delete_to_line_end": null,
               "enter_insert": null,
+              "enter_replace_mode": null,
               "find_backward": null,
               "find_forward": null,
               "insert_line_start": null,
@@ -4755,6 +4756,7 @@
             "delete_char": null,
             "delete_to_line_end": null,
             "enter_insert": null,
+            "enter_replace_mode": null,
             "find_backward": null,
             "find_forward": null,
             "insert_line_start": null,
@@ -5100,6 +5102,14 @@
           ],
           "description": "Enter insert mode at cursor (`i`)."
         },
+        "enter_replace_mode": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/KeybindingsSpec"
+            }
+          ],
+          "description": "Enter replace mode and overwrite characters under the cursor (`R`)."
+        },
         "find_backward": {
           "allOf": [
             {