fix(tui): hide empty reasoning summaries (#31652)

## Why

Reasoning summaries can contain an empty HTML comment placeholder, `<!--
-->`, when a generated summary part has no prose. The TUI treated that
placeholder as visible summary content, so it leaked into both the
completed conversation history and persisted transcript. Multi-part
empty summaries could also leave later generated status headings visible
as orphaned transcript content.

## What

- Strip the empty reasoning-summary placeholder at the shared
`ReasoningSummaryCell` boundary.
- Suppress the cell when removing placeholders leaves only generated
bold status headings.
- Cover the multi-part placeholder-only shape with a rendering
regression test for both normal history and transcript output.

## How to Test

1. Start Codex TUI with reasoning summaries visible, for example with
`hide_agent_reasoning = false` and `model_reasoning_summary =
"detailed"`.
2. Run a turn that receives an empty generated reasoning-summary part.
3. Confirm that the in-progress status heading may appear while the turn
is running, but the completed history and transcript contain neither
literal `<!-- -->` tokens nor orphaned reasoning headings.
4. Confirm that a reasoning summary with actual prose still renders
normally.

The model output is nondeterministic, so the targeted regression covers
the exact multi-part placeholder shape from the report:

- `just test -p codex-tui
reasoning_summary_block_hides_empty_html_comment_parts`

codex-rs/tui/src/chatwidget/replay.rs

@@ -113,13 +113,18 @@ impl ChatWidget {
                 summary, content, ..
             } => {
                 if from_replay {
-                    for delta in summary {
-                        self.on_agent_reasoning_delta(delta);
-                    }
-                    if self.config.show_raw_agent_reasoning {
-                        for delta in content {
-                            self.on_agent_reasoning_delta(delta);
+                    let reasoning_parts = summary.into_iter().chain(
+                        self.config
+                            .show_raw_agent_reasoning
+                            .then_some(content)
+                            .into_iter()
+                            .flatten(),
+                    );
+                    for (index, delta) in reasoning_parts.enumerate() {
+                        if index > 0 {
+                            self.on_reasoning_section_break();
                         }
+                        self.on_agent_reasoning_delta(delta);
                     }
                 }
                 self.on_agent_reasoning_final();