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.rs

@@ -630,6 +630,8 @@ pub(crate) struct ChatWidget {
     interrupts: InterruptManager,
     // Accumulates the current reasoning block text to extract a header
     reasoning_buffer: String,
+    // Caches the first completed bold header so later deltas do not rescan the whole block.
+    reasoning_header: Option<String>,
     // Preserves reasoning-summary part boundaries for transcript-only recording.
     reasoning_summary_parts: Vec<String>,
     status_state: StatusState,