Show unread sessions and acknowledge viewed transcripts in the picker (#58264)

## What changed

- Show a blue unread dot in both session picker layouts using backend read states.
- Acknowledge the transcript's observed revision after rendering it in a focused terminal, then update the unread indicator from the response.
- Key cached transcripts by backend and thread, reload them when unread revisions differ, and ignore canceled transcript results.
- Include turn errors in transcripts.

## Testing

Extend picker tests to cover unread indicators, selection colors, acknowledgments after rendering, and turn error display.

GitOrigin-RevId: edad152ed3f7086dfd3b2e8245d62febc5c9322b

codex-rs/tui/src/thread_transcript.rs

@@ -103,13 +103,19 @@ pub(crate) fn thread_to_transcript_cells(
         .turns
         .into_iter()
         .flat_map(|turn| {
-            thread_items_to_transcript_cells(
+            let mut cells = thread_items_to_transcript_cells(
                 thread_id,
                 &cwd,
                 turn.items.into_iter().map(|entry| entry.payload),
                 raw_reasoning_visibility,
                 config,
-            )
+            );
+            if let Some(error) = turn.error {
+                cells.push(Arc::new(crate::history_cell::new_error_event(
+                    error.message,
+                )));
+            }
+            cells
         })
         .collect::<TranscriptCells>();
     if cells.is_empty() {