fix(tui): hide empty reasoning summaries

[fcoury-oai]

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

[fcoury-oai]

@codex review

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[fcoury-oai]

@codex review

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Reviewed commit: 7879a4370f

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[fcoury-oai]

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Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: 1d82efac5e

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
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• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".