Fix noUncheckedIndexedAccess typecheck error in jsonc test

Array indexing returns number | undefined under strict TS config;
use a non-null assertion where the valid index is guaranteed by the test setup.

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

Author: sk-bryn

test/jsonc.test.ts

@@ -173,7 +173,8 @@ describe("scan", () => {
     const s = '{"a": 1} // tail';
     const { out, map } = scan(s);
     const closingBrace = out.indexOf("}");
-    expect(s[map[closingBrace]]).toBe("}");
+    const rawIdx = map[closingBrace]!;
+    expect(s[rawIdx]).toBe("}");
   });
 });