Avoid redundant TUI redraws while streaming (#34049)

## What changed

- Redraw assistant and plan stream tails only when a completed line changes the visible tail.
- Cache the first reasoning header and update the status indicator only when its visible state changes, while restoring the cached header if the indicator is recreated.
- Keep regular commit ticks able to clear stale stream tails.

## Testing

- Add coverage for unchanged partial deltas, changed newline-delimited tails, reasoning-header redraws, recreated status indicators, stale plan tails, and history-line draw scheduling.

GitOrigin-RevId: 9016175f97ef2db2edc2fd5ba13a4d731dc2d98c

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

@@ -739,6 +739,7 @@ impl ChatWidget {
                 );
                 if self.is_session_configured() {
                     self.reasoning_buffer.clear();
+                    self.reasoning_header = None;
                     self.reasoning_summary_parts.clear();
                     self.set_status_header(String::from("Working"));
                     self.submit_user_message(user_message);