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/src/context/mod.rs

@@ -54,6 +54,7 @@ pub(crate) use available_plugins_instructions::AvailablePluginsInstructions;
 pub(crate) use base_instructions::BaseInstructionsFragment;
 pub(crate) use codex_context_fragments::AdditionalContextDeveloperFragment;
 pub(crate) use codex_context_fragments::AdditionalContextUserFragment;
+pub use codex_context_fragments::AnsweredQuestion;
 pub use codex_context_fragments::ContextualUserFragment;
 pub(crate) use compaction_summary::CompactionSummary;
 pub(crate) use contextual_user_message::is_contextual_user_fragment;