feat(material/expansion): add border design tokens

Adds `expansion-container-outline-width` and
`expansion-container-outline-color` tokens to both M2 and M3 themes,
wired to CSS `border` properties on `.mat-expansion-panel`. Defaults are
`0` and `transparent` respectively, preserving existing visual behavior.

close #32878

Signed-off-by: Ruslan Lekhman <lekhman112@gmail.com>

Author: lekhmanrus

src/material/expansion/_m2-expansion.scss

@@ -11,12 +11,14 @@
   @return (
     base: (
       expansion-container-shape: 4px,
+      expansion-container-outline-width: 0,
       expansion-container-elevation-shadow: elevation.get-box-shadow(2),
       expansion-legacy-header-indicator-display: inline-block,
       expansion-header-indicator-display: none,
     ),
     color: (
       expansion-container-background-color: map.get($system, surface),
+      expansion-container-outline-color: transparent,
       expansion-container-text-color: map.get($system, on-surface),
       expansion-actions-divider-color: map.get($system, outline),
       expansion-header-hover-state-layer-color: m3-utils.color-with-opacity(

src/material/expansion/_m3-expansion.scss

@@ -15,13 +15,15 @@ $prefix: (mat, expansion);
   @return (
     base: (
       expansion-container-shape: 12px,
+      expansion-container-outline-width: 0,
       expansion-header-indicator-display: inline-block,
       expansion-legacy-header-indicator-display: none,
       expansion-container-elevation-shadow: elevation.get-box-shadow(2),
     ),
     color: (
       expansion-actions-divider-color: map.get($system, outline),
       expansion-container-background-color: map.get($system, surface),
+      expansion-container-outline-color: transparent,
       expansion-container-text-color: map.get($system, on-surface),
       expansion-header-description-color: map.get($system, on-surface-variant),
       expansion-header-disabled-state-text-color:

src/material/expansion/expansion-panel.scss

@@ -25,6 +25,9 @@ $fallbacks: m3-expansion.get-tokens();
   background: token-utils.slot(expansion-container-background-color, $fallbacks);
   color: token-utils.slot(expansion-container-text-color, $fallbacks);
   border-radius: token-utils.slot(expansion-container-shape, $fallbacks);
+  border-width: token-utils.slot(expansion-container-outline-width, $fallbacks);
+  border-color: token-utils.slot(expansion-container-outline-color, $fallbacks);
+  border-style: solid;
 
   &:not([class*='#{elevation.$prefix}']) {
     box-shadow: token-utils.slot(expansion-container-elevation-shadow, $fallbacks);