fix(tui): keep unified exec summary on working line (#10962)

## Problem
When unified-exec background sessions appear while the status indicator
is visible, the bottom pane can grow by one row to show a dedicated
footer line. That row insertion/removal makes the composer jump
vertically and produces visible jitter/flicker during streaming turns.

## Mental model
The bottom pane should expose one canonical background-exec summary
string, but it should surface that string in only one place at a time:
- if the status indicator row is visible, show the summary inline on
that row;
- if the status indicator row is hidden, show the summary as the
standalone unified-exec footer row.

This keeps status information visible while preserving a stable pane
height.

## Non-goals
This change does not alter unified-exec lifecycle, process tracking, or
`/ps` behavior. It does not redesign status text copy, spinner timing,
or interrupt handling semantics.

## Tradeoffs
Inlining the summary preserves layout stability and keeps interrupt
affordances in a fixed location, but it reduces horizontal space for
long status/detail text in narrow terminals. We accept that truncation
risk in exchange for removing vertical jitter and keeping the composer
anchored.

## Architecture
`UnifiedExecFooter` remains the source of truth for background-process
summary copy via `summary_text()`. `BottomPane` mirrors that text into
`StatusIndicatorWidget::update_inline_message()` whenever process state
changes or a status widget is created. Rendering enforces single-surface
output: the standalone footer row is skipped while status is present,
and the status row appends the summary after the elapsed/interrupt
segment.

## Documentation pass
Added non-functional docs/comments that make the new invariant explicit:
- status row owns inline summary when present;
- unified-exec footer row renders only when status row is absent;
- summary ordering keeps elapsed/interrupt affordance in a stable
position.

## Observability
No new telemetry or logs are introduced. The behavior is traceable
through:
- `BottomPane::set_unified_exec_processes()` for state updates,
- `BottomPane::sync_status_inline_message()` for status-row
synchronization,
- `StatusIndicatorWidget::render()` for final inline ordering.

## Tests
- Added
`bottom_pane::tests::unified_exec_summary_does_not_increase_height_when_status_visible`
to lock the no-height-growth invariant.
- Updated the unified-exec status restoration snapshot to match inline
rendering order.
- Validated with:
  - `just fmt`
  - `cargo test -p codex-tui --lib`

---------

Co-authored-by: Sayan Sisodiya <sayan@openai.com>

codex-rs/app-server-protocol/schema/json/ServerNotification.json

@@ -8197,4 +8197,4 @@
     }
   ],
   "title": "ServerNotification"
-}
+}