🧪 Add test for unexpected errors in evaluateDocument

Added a test in `src/engine/engine.test.ts` to mock evaluation failure and verify the engine safely catches it, nulls the value, populates the error string, and logs a warning.

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

src/engine/engine.test.ts

@@ -1,9 +1,11 @@
-import { describe, it, expect, beforeAll } from "vitest";
+import { describe, it, expect, beforeAll, vi } from "vitest";
 import { SumEngine } from "./index";
 import { qty, EvalError, XRefError } from "./types";
 import type { XRefResolution, EvalCtx } from "./evaluator";
+import * as evaluator from "./evaluator";
 import { evaluate } from "./evaluator";
 import { formatValue } from "./formatter";
+import { logger } from "../logger";
 
 let eng: SumEngine;
 
@@ -451,4 +453,21 @@ describe("evaluation errors", () => {
     } as unknown as EvalCtx;
     expect(() => evaluate({ k: "xref", sheet: "Sheet", key: "key" }, ctx)).toThrowError(XRefError);
   });
+
+  it("evaluateDocument captures unexpected errors", () => {
+    const mockEval = vi.spyOn(evaluator, "evaluate").mockImplementation(() => {
+      throw new Error("Boom");
+    });
+
+    const warnSpy = vi.spyOn(logger, "warn").mockImplementation(() => {});
+
+    const result = eng.evaluateDocument("1 + 1");
+
+    expect(result[0].error).toBe("Boom");
+    expect(result[0].value).toBeNull();
+    expect(warnSpy).toHaveBeenCalledWith("evaluate failed:", expect.any(Error));
+
+    warnSpy.mockRestore();
+    mockEval.mockRestore();
+  });
 });