[Expression] Fix missing border in mobile keypad (#2907)

## Summary:
Fix missing border in custom mobile keypad to make the experience consistent with desktop, and it will not look weird when above some text or other content

| Before | After |
| ------- | ----- |
| <img width="524" height="372" alt="image" src="https://github.com/user-attachments/assets/511af8c4-d631-437f-aae8-4cb2a1d89677" /> | <img width="294" height="588" alt="Screenshot 2025-09-17 at 4 00 36 PM" src="https://github.com/user-attachments/assets/1ab8b237-1101-4e1e-b1a6-b8fc29338692" /> |

Issue: LEMS-2924

## Test plan:
1. In mobile to Test Everything's Expression Widget `internal-courses/test-everything/test-everything-1/expression/a/expression-widget-for-level-access`
2. Confirm that the expression widget mobile keypad has visible border.

Author: ivyolamit

Reviewers: ivyolamit, jeremywiebe, #perseus, #lems

Required Reviewers:

Approved By: jeremywiebe

Checks: ✅ 10 checks were successful

Pull Request URL: #2907

Author: ivyolamit

.changeset/wicked-monkeys-heal.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/math-input": patch
+---
+
+Fix missing border in mobile keypad

packages/math-input/src/components/keypad/__tests__/__snapshots__/mobile-keypad.test.tsx.snap

@@ -3,15 +3,15 @@
 exports[`mobile keypad should not render the keypad when not active 1`] = `
 <div>
   <div
-    class="initial_4qg14c-o_O-keypadContainer_9ki71g"
+    class="initial_4qg14c-o_O-keypadContainer_1jsjomm"
   />
 </div>
 `;
 
 exports[`mobile keypad should render keypad when active 1`] = `
 <div>
   <div
-    class="initial_4qg14c-o_O-keypadContainer_9ki71g"
+    class="initial_4qg14c-o_O-keypadContainer_1jsjomm"
   >
     <div
       class="default_xu2jcg inlineStyles_6alflt"

packages/math-input/src/components/keypad/mobile-keypad-internals.tsx

@@ -1,3 +1,4 @@
+import {semanticColor} from "@khanacademy/wonder-blocks-tokens";
 import {StyleSheet} from "aphrodite";
 import * as React from "react";
 import ReactDOM from "react-dom";
@@ -243,6 +244,11 @@ const styles = StyleSheet.create({
         left: 0,
         right: 0,
         position: "fixed",
+        // Having the border will make the experience consistent with the desktop
+        // keypad which has a border.
+        borderStyle: "solid",
+        borderWidth: 1,
+        borderColor: semanticColor.core.border.neutral.subtle,
     },
 });