burst paste edge cases (#2683)

This PR fixes two edge cases in managing burst paste (mainly on power
shell).
Bugs:
- Needs an event key after paste to render the pasted items

> ChatComposer::flush_paste_burst_if_due() flushes on timeout. Called:
>     - Pre-render in App on TuiEvent::Draw.
>     - Via a delayed frame
>
BottomPane::request_redraw_in(ChatComposer::recommended_paste_flush_delay()).

- Parses two key events separately before starting parsing burst paste

> When threshold is crossed, pull preceding burst chars out of the
textarea and prepend to paste_burst_buffer, then keep buffering.

- Integrates with #2567 to bring image pasting to windows.

codex-rs/tui/src/chatwidget/tests.rs

@@ -164,6 +164,7 @@ fn make_chatwidget_manual() -> (
         has_input_focus: true,
         enhanced_keys_supported: false,
         placeholder_text: "Ask Codex to do anything".to_string(),
+        disable_paste_burst: false,
     });
     let widget = ChatWidget {
         app_event_tx,