Fix a type error in the hint truncation test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/hint.test.ts

@@ -99,7 +99,7 @@ describe("fitHintToBudget (D-WS2a truncation rule)", () => {
     expect(` ${hint}`.startsWith(kept)).toBe(true);
     // Ends on a whole term: the next character of the hint is a separator.
     expect(kept.endsWith(",")).toBe(false);
-    expect([",", " "]).toContain(` ${hint}`[kept.length]);
+    expect([",", " "]).toContain(` ${hint}`.charAt(kept.length));
   });
 
   test("is deterministic", () => {