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/constructor.rs

@@ -176,7 +176,7 @@ impl ChatWidget {
             newly_installed_marketplace_tab_id: None,
             interrupts: InterruptManager::new(),
             reasoning_buffer: String::new(),
-            full_reasoning_buffer: String::new(),
+            reasoning_summary_parts: Vec::new(),
             status_state: StatusState::default(),
             review: ReviewState::default(),
             active_hook_cell: None,