test: limit ts-jest diagnostics to test files

Type errors in transformed dependencies (e.g. @edx/frontend-component-header,
included via transformIgnorePatterns) were failing fresh-cache CI runs while
passing locally due to ts-jest cache. Restrict diagnostics to test files only.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: arbrandes

jest.config.ts

@@ -1,6 +1,6 @@
 const { createConfig } = require('@openedx/frontend-build');
 
-module.exports = createConfig('jest', {
+const mergedConfig = createConfig('jest', {
   // setupFilesAfterEnv is used after the jest environment has been loaded. In general this is what you want.
   // If you want to add config BEFORE jest loads, use setupFiles instead.
   setupFilesAfterEnv: [
@@ -16,4 +16,17 @@ module.exports = createConfig('jest', {
   }
 });
 
+// Limit ts-jest diagnostics to test files so type errors in transformed
+// dependencies (included via transformIgnorePatterns) don't fail the run.
+mergedConfig.transform['^.+\\.[tj]sx?$'] = [
+  'ts-jest',
+  {
+    diagnostics: {
+      exclude: ['!**/*.test.*'],
+    },
+  },
+];
+
+module.exports = mergedConfig;
+
 export {};