Add missing error test for invalid IANA timezone in resolveZone

Tested `resolveZone` boundary conditions by verifying that invalid strings trigger the internal Intl.DateTimeFormat error catch and correctly evaluate to null.

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

src/engine/datetime.test.ts

@@ -78,6 +78,7 @@ describe("datetime", () => {
     it("should return null for unknown timezones", () => {
       expect(resolveZone(["Unknown", "City"])).toBeNull();
       expect(resolveZone(["Invalid/Timezone"])).toBeNull();
+      expect(resolveZone(["Invalid", "Zone"])).toBeNull();
     });
   });