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.

src/ast/compiler/type_checker.rs

@@ -1513,16 +1513,8 @@ impl TypeChecker {
                     self.check_node(args, ctx, diag)
                 };
 
-                if let Some(expected_ty) = &ctx.current_params_ty
-                    && !expected_ty.is_assignable_from(&args_typed.ty)
-                {
-                    diag.push_error(
-                        format!(
-                            "Type mismatch in 'again' call: expected {}, but got {}",
-                            expected_ty.display_compact(), args_typed.ty.display_compact()
-                        ),
-                        Some(node.identity.clone()),
-                    );
+                if let Some(expected_ty) = &ctx.current_params_ty {
+                    self.unify(expected_ty.clone(), args_typed.ty.clone(), diag);
                 }
 
                 (