Use consistent TUI input placeholders (#37360)

## What changed

- Show `Ask Codex to do anything` in the main conversation input.
- Show `Ask a follow-up question` in side conversation inputs.
- Remove randomized example prompt selection and update the side conversation snapshots.

GitOrigin-RevId: 2f6d310cf80bfa9743b7695621892a821eda4a18

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

@@ -206,12 +206,6 @@ pub(super) async fn make_chatwidget_manual_with_auth(
     let mut widget = ChatWidget::new_with_op_target(common, super::CodexOpTarget::Direct(op_tx));
     widget.transcript.active_cell = None;
     widget.transcript.active_cell_revision = 0;
-    widget.normal_placeholder_text = "Ask Codex to do anything".to_string();
-    widget.side_placeholder_text =
-        "Check recently modified functions for compatibility".to_string();
-    widget
-        .bottom_pane
-        .set_placeholder_text(widget.normal_placeholder_text.clone());
     widget.set_model(&resolved_model);
     (widget, rx, op_rx)
 }