sessions: fix Customizations single-entry width overflow (#315125)

The single-entry sidebar 'Customizations' button had `width: 100%` set on
both the link-button container and the inner button. Combined with the
inherited `margin: 0 10px` from the per-section rule, this overflowed the
panel by 20px and was visible as a misaligned focus outline that extended
past the panel edge.

Drop the explicit widths and let the parent's default `align-items: stretch`
plus the inherited margins size the entry, matching the per-section items.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: joshspicer

src/vs/sessions/contrib/sessions/browser/media/customizationsToolbar.css

@@ -173,14 +173,9 @@
 	padding: 6px 0 10px 0;
 }
 
-/* Override the shared `flex: 1` on the link-button container so the single
-	entry sizes to its content height instead of stretching to fill the
-	flex-column toolbar. */
+/* Disable the shared `flex: 1` so the single entry doesn't stretch to fill
+	the flex-column toolbar. Width is handled by the inherited `margin: 0 10px`
+	and the parent's default `align-items: stretch`. */
 .ai-customization-toolbar.single-entry .customization-link-button-container {
 	flex: none;
-	width: 100%;
-}
-
-.ai-customization-toolbar.single-entry .customization-link-button {
-	width: 100%;
 }