Show debug-mode message for widget rebuilds in profile mode (#9755)

When running in profile mode, the Performance panel showed a
disabled 'Count widget builds' checkbox which was misleading.
Widget rebuild counts rely on debugOnRebuildDirtyWidget which
is only available in debug mode.

This change:
- Removes the checkbox when running in profile mode
- Shows a clear message: 'Rebuild information is not available
  for this frame. Widget rebuild counts are only available when
  running an app in debug-mode.'
- Adds widget tests for both profile mode and debug mode behavior

Fixes #9730


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Co-authored-by: Sam Rawlins <srawlins@google.com>