feat(tui): render markdown tables in app style [1 of 2] (#24489)

## Stack

- **Current: #24489 [1 of 2]** - render markdown tables in app style.
- **Stacked follow-up: #24636 [2 of 2]** - render cramped markdown
tables as key/value records.

## Why

Markdown tables currently render as boxed terminal grids, which gives
ordinary assistant output a heavier visual treatment than surrounding
rich text. This row-separated layout is the best match for how the App
renders tables, while accented headers remain distinguishable even when
a terminal font renders bold subtly.

<table>
<tr><td>
<p align="center">Codex CLI - Before</p>
<img width="1722" height="742" alt="CleanShot 2026-05-25 at 18 46 17"
src="https://github.com/user-attachments/assets/f673d92a-ebd8-46e2-b414-3d985e41b6a4"
/>
</td></tr>
<tr><td>
<p align="center">Codex CLI - After</p>
<img width="1720" height="957" alt="image"
src="https://github.com/user-attachments/assets/36a3d331-bea1-439b-b5be-e97b0731bd6f"
/>
</td></tr>
<tr><td>
<p align="center">Codex App</p>
<img width="979" height="1293" alt="CleanShot 2026-05-25 at 18 45 04"
src="https://github.com/user-attachments/assets/7d97cae0-9256-4f6e-a4b3-8b8f22b0d901"
/>
</td></tr>
</table>

## What Changed

- Render markdown tables as padded, aligned rows without an enclosing
box.
- Style table headers with the active syntax-theme accent plus bold
text, while keeping separators low contrast and theme-aware.
- Use a segmented heavy header rule and thin body-row rules, preserving
wrapping, narrow-width fallback, streaming parity, and rich-history
rendering.
- Update focused assertions and snapshots for the final table layout.

## How to Test

1. Render a markdown table in the TUI with several rows and columns.
2. Confirm the header uses the active theme accent, rows use
one-character interior padding, and the table has no enclosing box.
3. Confirm the header is followed by segmented `━` rules and multiple
body rows are separated by muted segmented `─` rules.
4. Render the same table while streaming and in history/raw-mode
toggles; the final rich layout should remain stable.
5. Render a narrow table with long content and verify wrapping or pipe
fallback does not overflow.

## Validation

- `just test -p codex-tui table`
- `just test -p codex-tui streaming::controller::tests`
- `just argument-comment-lint-from-source -p codex-tui -- --all-targets`
- `just fix -p codex-tui`
- `just fmt`

`just test -p codex-tui` was also run after accepting the snapshots; it
fails only in the unrelated existing guardian app tests
`update_feature_flags_disabling_guardian_clears_review_policy_and_restores_default`
and
`update_feature_flags_disabling_guardian_clears_manual_review_policy_without_history`.

codex-rs/tui/src/chatwidget/tests/status_and_layout.rs

@@ -393,8 +393,8 @@ async fn completed_plan_table_tail_skips_provisional_history_insert() {
 
     assert!(saw_source_backed_plan, "expected source-backed plan insert");
     assert!(
-        rendered_plan.contains('│') || rendered_plan.contains('┌'),
-        "expected completed plan table to render as a boxed table, got: {rendered_plan:?}"
+        rendered_plan.contains('━'),
+        "expected completed plan table to render with separators, got: {rendered_plan:?}"
     );
     assert!(
         !saw_stream_plan,