test: add unit tests for validString

test/valid-string.test.ts

@@ -0,0 +1,17 @@
+import { validString } from "../app/client/api";
+
+describe("validString", () => {
+  test("returns true for a non-empty string", () => {
+    expect(validString("a")).toBe(true);
+    expect(validString("  padded  ")).toBe(true);
+  });
+
+  test("returns false for an empty string", () => {
+    expect(validString("")).toBe(false);
+  });
+
+  test("returns false for null / undefined input", () => {
+    expect(validString(undefined as any)).toBe(false);
+    expect(validString(null as any)).toBe(false);
+  });
+});