FN-5934: replace undefined dashboard spacing token

Replace dashboard CSS uses of the undefined --space-2xs token with supported spacing values.

- swap var(--space-2xs) for var(--space-xs) in confirm dialog, merge advance notice, and PR checks styles
- add a dashboard test that scans component stylesheets for undefined --space-2xs references
- assert shared token sources continue to omit an intentional --space-2xs definition

Files changed:
 .../app/__tests__/space-token-defined.test.ts      | 45 ++++++++++++++++++++++
 .../dashboard/app/components/ConfirmDialog.css     |  2 +-
 .../app/components/MergeAdvanceNotice.css          |  8 ++--
 packages/dashboard/app/components/PrChecksList.css |  2 +-
 4 files changed, 51 insertions(+), 6 deletions(-)

Fusion-Task-Id: FN-5934

Fusion-Task-Lineage: aa77df19-3f76-4ede-aa42-0396230edde5

packages/dashboard/app/components/PrChecksList.css

@@ -111,7 +111,7 @@
   border-radius: var(--radius-sm);
   color: var(--color-error);
   justify-content: center;
-  padding: var(--space-2xs) var(--space-sm);
+  padding: var(--space-xs) var(--space-sm);
 }
 
 .pr-checks__details-link--failing:hover {