feat(wad_ray_math_tests): add overflow test for ray_mul_down

- Change:
  - Add `test_ray_mul_down_overflow`: `b = 1 RAY`, `a = U256_MAX / b + 1`
  - Expect abort `EOVERFLOW` (location = `aave_math::wad_ray_math`)
- Preserve:
  - Existing tests and helpers
  - Public API and function names
- Rationale:
  - Validate new overflow guard for `a*b` path
  - Align with existing overflow tests for `ray_mul`/`ray_mul_up`
- Impact:
  - Test coverage improved
  - No behavior change in non-test code

Author: matchv

aave-core/aave-math/tests/wad_ray_math_tests.move

@@ -454,4 +454,13 @@ module aave_math::wad_ray_math_tests {
         let b = 1;
         ray_div_down(a, b);
     }
+
+    #[test]
+    #[expected_failure(abort_code = EOVERFLOW, location = aave_math::wad_ray_math)]
+    fun test_ray_mul_down_overflow() {
+        // a * b overflows at a = floor(U256_MAX / b) + 1 (choose b > 0)
+        let b = 1000000000000000000000000000; // 1 RAY
+        let a = get_u256_max_for_testing() / b + 1;
+        ray_mul_down(a, b);
+    }
 }