tui: separator above final agent message (#4324)

Adds a separator line before the final agent message

<img width="1011" height="884" alt="Screenshot 2025-09-26 at 4 55 01 PM"
src="https://github.com/user-attachments/assets/7c91adbf-6035-4578-8b88-a6921f11bcbc"
/>

codex-rs/tui/src/history_cell.rs

@@ -1090,6 +1090,40 @@ pub(crate) fn new_reasoning_summary_block(
     Box::new(new_reasoning_block(full_reasoning_buffer, config))
 }
 
+#[derive(Debug)]
+pub struct FinalMessageSeparator {
+    elapsed_seconds: Option<u64>,
+}
+impl FinalMessageSeparator {
+    pub(crate) fn new(elapsed_seconds: Option<u64>) -> Self {
+        Self { elapsed_seconds }
+    }
+}
+impl HistoryCell for FinalMessageSeparator {
+    fn display_lines(&self, width: u16) -> Vec<Line<'static>> {
+        let elapsed_seconds = self
+            .elapsed_seconds
+            .map(super::status_indicator_widget::fmt_elapsed_compact);
+        if let Some(elapsed_seconds) = elapsed_seconds {
+            let worked_for = format!("─ Worked for {elapsed_seconds} ─");
+            let worked_for_width = worked_for.width();
+            vec![
+                Line::from_iter([
+                    worked_for,
+                    "─".repeat((width as usize).saturating_sub(worked_for_width)),
+                ])
+                .dim(),
+            ]
+        } else {
+            vec![Line::from_iter(["─".repeat(width as usize).dim()])]
+        }
+    }
+
+    fn transcript_lines(&self) -> Vec<Line<'static>> {
+        vec![]
+    }
+}
+
 fn format_mcp_invocation<'a>(invocation: McpInvocation) -> Line<'a> {
     let args_str = invocation
         .arguments