Add TUI building blocks for inline async question editing (#42889)

## What changed

- Introduce an async-question editor component with per-question drafts, navigation, replay deduplication, and submit or queue handling using the shared composer.
- Add bounded `AnsweredQuestion` framing that truncates question text at a UTF-8 boundary and flattens line breaks before prepending it to an answer.
- Add `prompt_stack_back` and `skip_question` keybinding actions and the `tui.question_esc_back` setting. Normalize `Ctrl+]` and `Ctrl+5` for key matching and conflict detection, and let default question shortcuts yield to explicit bindings.
- Flush buffered typing in both the main composer and the covering view so background input cannot keep the draw loop waiting.

The async-question component is not yet connected to the TUI event flow.

## Testing

Add regression coverage for bounded Unicode question framing, question-shortcut conflicts, and paste-buffer flushing in background and covering editors.

GitOrigin-RevId: 4098043cf588ddafc05c27505645495edd690cb9

codex-rs/core/config.schema.json

@@ -3889,6 +3889,8 @@
               "interrupt_turn": null,
               "next_permission_mode": null,
               "previous_permission_mode": null,
+              "prompt_stack_back": null,
+              "skip_question": null,
               "toggle_voice_mute": null
             },
             "composer": {
@@ -4085,6 +4087,11 @@
           "default": "composer",
           "description": "Where the terminal pet should anchor vertically.\n\nDefaults to `composer`, which follows the current TUI composer viewport."
         },
+        "question_esc_back": {
+          "default": true,
+          "description": "Escape returns from async questions to the composer, preserving the answer draft.",
+          "type": "boolean"
+        },
         "raw_output_mode": {
           "default": false,
           "description": "Start the TUI in raw scrollback mode for copy-friendly transcript output. Defaults to `false`.",
@@ -4303,7 +4310,7 @@
               "$ref": "#/definitions/KeybindingsSpec"
             }
           ],
-          "description": "Edit the most recently queued message."
+          "description": "Move up through pending async questions, then edit the most recently queued message."
         },
         "increase_reasoning_effort": {
           "allOf": [
@@ -4337,6 +4344,22 @@
           ],
           "description": "Switch to the previous available permission mode."
         },
+        "prompt_stack_back": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/KeybindingsSpec"
+            }
+          ],
+          "description": "Move back through pending async questions toward the composer."
+        },
+        "skip_question": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/KeybindingsSpec"
+            }
+          ],
+          "description": "Skip the focused question."
+        },
         "toggle_voice_mute": {
           "allOf": [
             {
@@ -4690,6 +4713,8 @@
             "interrupt_turn": null,
             "next_permission_mode": null,
             "previous_permission_mode": null,
+            "prompt_stack_back": null,
+            "skip_question": null,
             "toggle_voice_mute": null
           }
         },