Refactor 'again' type check to use unify

This change replaces a manual type check for the `again` function with a
call to the generic `unify` function. This improves consistency and
leverages existing type-checking logic.

A new test case `test_again_type_mismatch` has been added to
specifically verify that type mismatches in `again` calls are caught at
compile time, preventing potential infinite loops.

tests/error_recovery.rs

@@ -160,3 +160,17 @@ fn test_error_recovery_multiple_errors() {
             .any(|m| m.contains("no matching overload"))
     );
 }
+
+#[test]
+fn test_again_type_mismatch() {
+    let env = Environment::new();
+    // (again "text") should fail: parameter n is int (from call site), not string.
+    // Without this check, the program would loop forever.
+    let result = env.run_script(
+        r#"(do (def f (fn [n] (if (= n 0) n (again "text")))) (f 3))"#,
+    );
+    assert!(
+        result.is_err(),
+        "again with wrong type should be a compile error, not an infinite loop"
+    );
+}