fix(material/dialog): reserve room for touch targets in scrollable content

`mat-dialog-content` scrolls, and its block-end padding is zero when the dialog has an
actions row. Buttons, checkboxes and radios center a 48px touch target on a 40px control,
and slide toggles center one on a 32px switch, so the target overhangs the control by 4-8px.
An overhanging target at the end of the content is scrollable overflow, so a dialog whose
content ends in one of those controls paints a scrollbar over content that fits.

Reserve the overhang in the block-end padding instead. This keeps the touch targets at their
accessible size, which the alternatives do not: disabling them
(`touch-target-display: none`) shrinks the tap area, and anchoring them so they only overhang
the block-start edge still left 4px of overflow in WebKit in local testing.

Verified with a stock Material app in Chromium, Firefox and WebKit. A dialog with a trailing
checkbox, radio, slide toggle or button goes from 4px, 4px, 8px and 4px of scrollable
overflow respectively to 0 in all three engines. Dialogs with actions grow by 8px, which is
the trade-off for not touching the tap areas.

Fixes #29164.

Author: ManicardiFrancesco

src/material/dialog/_m3-dialog.scss

@@ -23,7 +23,9 @@
       dialog-container-small-max-width: calc(100vw - 32px),
       dialog-content-padding: 20px 24px,
       dialog-headline-padding: 6px 24px 13px,
-      dialog-with-actions-content-padding: 20px 24px 0,
+      // The block-end value reserves room for the touch targets of any controls at the end of
+      // the content, which would otherwise be scrollable overflow. See dialog.scss.
+      dialog-with-actions-content-padding: 20px 24px 8px,
     ),
     color: (
       dialog-container-color: map.get($system, surface),

src/material/dialog/dialog.scss

@@ -191,8 +191,15 @@ $fallbacks: m3-dialog.get-tokens();
 
   // Note: we can achieve this with a `:has` selector, but it results in an
   // increased specificity which breaks a lot of internal clients.
+  //
+  // The block-end padding reserves room for the touch targets of the controls the content may
+  // contain. Buttons, checkboxes and radios center a 48px touch target on a 40px control and
+  // slide toggles center one on a 32px switch, so the target overhangs the control by up to
+  // 8px. Since this element scrolls, an overhanging target at the end of the content is
+  // scrollable overflow, and the dialog shows a scrollbar over content that fits. Reserving
+  // the overhang fixes it without shrinking the targets below their accessible size.
   .mat-mdc-dialog-container-with-actions & {
-    padding: token-utils.slot(dialog-with-actions-content-padding, $fallbacks, 20px 24px 0);
+    padding: token-utils.slot(dialog-with-actions-content-padding, $fallbacks, 20px 24px 8px);
   }
 
   .mat-mdc-dialog-container .mat-mdc-dialog-title + & {