Add paste summarization to Codex TUI (#1549)

## Summary
- introduce `Paste` event to avoid per-character paste handling
- collapse large pasted blocks to `[Pasted Content X lines]`
- store the real text so submission still includes it
- wire paste handling through `App`, `ChatWidget`, `BottomPane`, and
`ChatComposer`

## Testing
- `cargo test -p codex-tui`


------
https://chatgpt.com/codex/tasks/task_i_6871e24abf80832184d1f3ca0c61a5ee


https://github.com/user-attachments/assets/eda7412f-da30-4474-9f7c-96b49d48fbf8

codex-rs/tui/src/chatwidget.rs

@@ -174,6 +174,12 @@ impl ChatWidget<'_> {
         }
     }
 
+    pub(crate) fn handle_paste(&mut self, text: String) {
+        if matches!(self.input_focus, InputFocus::BottomPane) {
+            self.bottom_pane.handle_paste(text);
+        }
+    }
+
     fn submit_user_message(&mut self, user_message: UserMessage) {
         let UserMessage { text, image_paths } = user_message;
         let mut items: Vec<InputItem> = Vec::new();