test(frontend): add unit spec for extractErrorCode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/shared/api.server.spec.ts

@@ -0,0 +1,20 @@
+import { describe, it, expect } from 'vitest';
+import { extractErrorCode } from './api.server';
+
+describe('extractErrorCode', () => {
+	it('returns the code string when error has a code property', () => {
+		expect(extractErrorCode({ code: 'DOCUMENT_NOT_FOUND' })).toBe('DOCUMENT_NOT_FOUND');
+	});
+	it('returns undefined when error is undefined', () => {
+		expect(extractErrorCode(undefined)).toBeUndefined();
+	});
+	it('returns undefined when error is null', () => {
+		expect(extractErrorCode(null)).toBeUndefined();
+	});
+	it('returns undefined when error is a plain string', () => {
+		expect(extractErrorCode('oops')).toBeUndefined();
+	});
+	it('returns undefined when error object has no code property', () => {
+		expect(extractErrorCode({ message: 'fail' })).toBeUndefined();
+	});
+});