add pulsing dot loading state (#4736)

## Description 
Changes default CLI spinner to pulsing dot


https://github.com/user-attachments/assets/b81225d6-6655-4ead-8cb1-d6568a603d5b

## Tests
Passes CI

---------

Co-authored-by: Fouad Matin <fouad@openai.com>

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__exploring_step1_start_ls.snap

@@ -2,5 +2,5 @@
 source: tui/src/chatwidget/tests.rs
 expression: blob1
 ---
-⠋ Exploring
+• Exploring
   └ List ls -la

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__exploring_step3_start_cat_foo.snap

@@ -2,6 +2,6 @@
 source: tui/src/chatwidget/tests.rs
 expression: blob3
 ---
-⠋ Exploring
+• Exploring
   └ List ls -la
     Read foo.txt

codex-rs/tui/src/exec_cell/render.rs

@@ -116,12 +116,10 @@ pub(crate) fn output_lines(
 }
 
 pub(crate) fn spinner(start_time: Option<Instant>) -> Span<'static> {
-    const FRAMES: &[char] = &['⠋', '⠙', '⠹', '⠸', '⠼', '⠴', '⠦', '⠧', '⠇', '⠏'];
-    let idx = start_time
-        .map(|st| ((st.elapsed().as_millis() / 100) as usize) % FRAMES.len())
-        .unwrap_or(0);
-    let ch = FRAMES[idx];
-    ch.to_string().into()
+    let blink_on = start_time
+        .map(|st| ((st.elapsed().as_millis() / 600) % 2) == 0)
+        .unwrap_or(false);
+    if blink_on { "•".into() } else { "◦".dim() }
 }
 
 impl HistoryCell for ExecCell {

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__active_mcp_tool_call_snapshot.snap

@@ -3,4 +3,4 @@ source: tui/src/history_cell.rs
 assertion_line: 1740
 expression: rendered
 ---
-⠋ Calling search.find_docs({"query":"ratatui styling","limit":3})
+• Calling search.find_docs({"query":"ratatui styling","limit":3})

codex-rs/tui/src/status_indicator_widget.rs

@@ -134,12 +134,16 @@ impl StatusIndicatorWidget {
         self.frame_requester.schedule_frame();
     }
 
-    fn elapsed_seconds_at(&self, now: Instant) -> u64 {
+    fn elapsed_duration_at(&self, now: Instant) -> Duration {
         let mut elapsed = self.elapsed_running;
         if !self.is_paused {
             elapsed += now.saturating_duration_since(self.last_resume_at);
         }
-        elapsed.as_secs()
+        elapsed
+    }
+
+    fn elapsed_seconds_at(&self, now: Instant) -> u64 {
+        self.elapsed_duration_at(now).as_secs()
     }
 
     pub fn elapsed_seconds(&self) -> u64 {
@@ -156,11 +160,18 @@ impl WidgetRef for StatusIndicatorWidget {
         // Schedule next animation frame.
         self.frame_requester
             .schedule_frame_in(Duration::from_millis(32));
-        let elapsed = self.elapsed_seconds();
-        let pretty_elapsed = fmt_elapsed_compact(elapsed);
+        let now = Instant::now();
+        let elapsed_duration = self.elapsed_duration_at(now);
+        let pretty_elapsed = fmt_elapsed_compact(elapsed_duration.as_secs());
+        let blink_on = (elapsed_duration.as_millis() / 600).is_multiple_of(2);
 
         // Plain rendering: no borders or padding so the live cell is visually indistinguishable from terminal scrollback.
-        let mut spans = vec!["• ".dim()];
+        let mut spans = Vec::with_capacity(5);
+        if blink_on {
+            spans.push("• ".into());
+        } else {
+            spans.push("◦ ".dim());
+        }
         spans.extend(shimmer_spans(&self.header));
         spans.extend(vec![
             " ".into(),