Round-2 Copilot review on #14968 caught two leftover spots that didn't
fully respect per-section overrides:
- messageLine.tsx (trail branch): the previous fix gated on
`SECTION_NAMES.some(...)`, which stayed true whenever any section was
visible. With `thinking: 'expanded'` as the new built-in default,
that meant `display.sections.tools: hidden` left an empty wrapper Box
alive for trail messages. Now gates on the actual content-bearing
sections for a trail message — `tools` OR `activity` — so a
tools-hidden config drops the wrapper cleanly.
- messageLine.tsx (showDetails): still keyed off the global
`detailsMode !== 'hidden'`, so per-section overrides like
`sections.thinking: expanded` couldn't escape global hidden for
assistant messages with reasoning + tool metadata. Recomputed via
resolved per-section modes (`thinkingMode`/`toolsMode`).
- types.ts: rewrote the SectionVisibility doc comment to reflect the
actual resolution order (explicit override → SECTION_DEFAULTS →
global), so the docstring stops claiming "missing keys fall back to
the global mode" when SECTION_DEFAULTS now layers in between.
All three lookups (thinking/tools/activity) are computed once at the
top of MessageLine and shared by every branch.
6604e94c75