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`


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


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

codex-rs/tui/src/app_event.rs

@@ -12,6 +12,9 @@ pub(crate) enum AppEvent {
 
     KeyEvent(KeyEvent),
 
+    /// Text pasted from the terminal clipboard.
+    Paste(String),
+
     /// Scroll event with a value representing the "scroll delta" as the net
     /// scroll up/down events within a short time window.
     Scroll(i32),