Show completed hook warnings in TUI headers (#34416)

## What changed

- Render the first line of a completed hook warning in the hook header using
  `says:`, with any remaining lines indented below it.
- Keep the existing hook header when no warning is present and avoid rendering
  warning entries a second time in the output body.
- Update hook event snapshots and the multiline warning test for the new layout.

GitOrigin-RevId: 291b556daf3b6fb64a99b090a6df1a222a18a736

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__post_tool_use_hook_events_render_snapshot.snap

@@ -2,6 +2,5 @@
 source: tui/src/chatwidget/tests/helpers.rs
 expression: combined
 ---
-• PostToolUse hook (completed)
-  warning: Heads up from the hook
+• PostToolUse (completed) says: Heads up from the hook
   hook context: Remember the startup checklist.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__pre_tool_use_hook_events_render_snapshot.snap

@@ -2,6 +2,5 @@
 source: tui/src/chatwidget/tests/helpers.rs
 expression: combined
 ---
-• PreToolUse hook (completed)
-  warning: Heads up from the hook
+• PreToolUse (completed) says: Heads up from the hook
   hook context: Remember the startup checklist.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__session_start_hook_events_render_snapshot.snap

@@ -2,6 +2,5 @@
 source: tui/src/chatwidget/tests/helpers.rs
 expression: combined
 ---
-• SessionStart hook (completed)
-  warning: Heads up from the hook
+• SessionStart (completed) says: Heads up from the hook
   hook context: Remember the startup checklist.

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__user_prompt_submit_app_server_hook_notifications_render_snapshot.snap

@@ -2,6 +2,5 @@
 source: tui/src/chatwidget/tests/status_and_layout.rs
 expression: combined
 ---
-• UserPromptSubmit hook (stopped)
-  warning: go-workflow must start from PlanMode
+• UserPromptSubmit (stopped) says: go-workflow must start from PlanMode
   stop: prompt blocked