Clear copy pill background and add snapshot test (#8777)

### Motivation
- Fix a visual bug where transcript text could bleed through the
on-screen copy "pill" overlay.
- Ensure the copy affordance fully covers the underlying buffer so the
pill background is solid and consistent with styling.
- Document the approach in-code to make the background-clearing
rationale explicit.

### Description
- Clear the pill area before drawing by iterating `Rect::positions()`
and calling `cell.set_symbol(" ")` and `cell.set_style(base_style)` in
`render_copy_pill` in `transcript_copy_ui.rs`.
- Added an explanatory comment for why the pill background is explicitly
cleared.
- Added a unit test `copy_pill_clears_background` and committed the
corresponding snapshot file to validate the rendering behavior.

### Testing
- Ran `just fmt` (formatting completed; non-blocking environment warning
may appear).
- Ran `just fix -p codex-tui2` to apply lints/fixes (completed). 
- Ran `cargo test -p codex-tui2` and all tests passed (snapshot updated
and tests succeeded).

------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_695c9b23e9b8832997d5a457c4d83410)

codex-rs/tui2/src/snapshots/codex_tui2__transcript_copy_ui__tests__copy_pill_clears_background.snap

@@ -0,0 +1,7 @@
+---
+source: tui2/src/transcript_copy_ui.rs
+expression: rendered
+---
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+xxxxxxxxxx ⧉ copy ctrl + shift + c xxxxx

codex-rs/tui2/src/transcript_copy_ui.rs

@@ -287,6 +287,13 @@ impl TranscriptCopyUi {
         let icon_style = base_style.add_modifier(Modifier::BOLD).fg(Color::LightCyan);
         let bold_style = base_style.add_modifier(Modifier::BOLD);
 
+        // Clear background so underlying text doesn't bleed through the pill.
+        for position in pill_area.positions() {
+            let cell = &mut buf[position];
+            cell.set_symbol(" ");
+            cell.set_style(base_style);
+        }
+
         let mut spans: Vec<Span<'static>> = vec![
             Span::styled(" ", base_style),
             Span::styled("⧉", icon_style),
@@ -305,6 +312,7 @@ impl TranscriptCopyUi {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use insta::assert_snapshot;
     use ratatui::buffer::Buffer;
 
     fn buf_to_string(buf: &Buffer, area: Rect) -> String {
@@ -359,4 +367,21 @@ mod tests {
         assert_eq!(rect.y, 1);
         assert!(ui.hit_test(rect.x, rect.y));
     }
+
+    #[test]
+    fn copy_pill_clears_background() {
+        let area = Rect::new(0, 0, 40, 3);
+        let mut buf = Buffer::empty(area);
+        for y in 0..area.height {
+            for x in 0..area.width {
+                buf[(x, y)].set_symbol("x");
+            }
+        }
+
+        let mut ui = TranscriptCopyUi::new_with_shortcut(CopySelectionShortcut::CtrlShiftC);
+        ui.render_copy_pill(area, &mut buf, (1, 2), (1, 6), 0, 3);
+
+        let rendered = buf_to_string(&buf, area);
+        assert_snapshot!("copy_pill_clears_background", rendered);
+    }
 }