fix(ui): consistent padding for collapsed/expanded package accordions (#4426)

When viewing affected packages on vulnerability pages, the last package
in an ecosystem had different bottom padding depending on whether it was
collapsed or expanded:

<img width="1777" height="849" alt="befafter"
src="https://github.com/user-attachments/assets/12b4af46-6cd4-4a13-b8bb-bad533c19742"
/>

This was caused by the CSS rule `margin-bottom` being applied to
`.package-details-card` inside `.package-accordion--last`. When the
accordion was collapsed, this element was hidden, so the margin didn't
apply.

The padding rule is now applied on the last accordion element instead of
the hidden child element.

Author: ashmod

gcp/website/frontend3/src/styles.scss

@@ -1071,7 +1071,7 @@ dl.vulnerability-details,
     }
 
     $last-ecosystem-border-offset: 12px;
-    $last-ecosystem-border-gap: 8px;
+    $last-ecosystem-border-gap: 16px;
     $last-ecosystem-border-thickness: 1px;
 
     .ecosystem-content-panel--last {
@@ -1110,10 +1110,7 @@ dl.vulnerability-details,
 
     .package-accordion--last {
       margin-bottom: 0;
-
-      .package-details-card {
-        margin-bottom: $last-ecosystem-border-gap;
-      }
+      padding-bottom: $last-ecosystem-border-gap;
     }
 
     .package-accordion h3.package-name-title {