Refactor github mock to use Context class

Co-Authored-By: Claude <[email protected]>

Author: yuzurihaaa

__mocks__/@actions/github.js

@@ -0,0 +1,3 @@
+import {Context} from '@actions/github/lib/context';
+
+export const context = new Context();

__tests__/mock/github-mock.ts

@@ -9,6 +9,6 @@ module.exports = {
   },
   verbose: true,
   moduleNameMapper: {
-    '^@actions/github$': '<rootDir>/__mocks__/@actions/github.js'
+    '^@actions/github$': '<rootDir>/__tests__/mock/github-mock.ts'
   }
 }