feat(core): integrate Detector into DefaultRoleManager and add unit tests

**Background:**
We need to add a hook point for cycle detection into jcasbin, while following the principles of “minimal intrusion” and “loose coupling.” `DefaultRoleManager`’s `addLink` method is the only place that updates the inheritance relationship
github.com
.

**Tasks:**

1. Add an optional `Detector` field in `DefaultRoleManager`:

private Detector detector;

public void setDetector(Detector detector) {
this.detector = detector;
}

The default value is `null` to maintain backward compatibility.
2. Modify `addLink(String name1, String name2, String... domain)`: after successfully adding the parent-child relationship, if `detector` is not `null`, call `detector.check(this)`.

* If a non-empty error message is returned, immediately roll back the current `addRole` operation (call `removeRole` or delete that link), and throw an `IllegalArgumentException` containing the error description.
3. Create `src/test/java/org/casbin/jcasbin/main/DetectorTest.java`:

* Construct a `DefaultRoleManager`, inject a `DefaultDetector`, add some valid inheritance chains, and assert that no exception is thrown.
* Construct a chain containing a cycle, e.g., A→B→C→A, and assert that when adding the third link, an exception is thrown.
* Assert that after rollback, the state no longer contains the illegal link.
* If necessary, add more boundary tests (e.g., self-loop, complex graphs).
4. In tests, do not depend on `Enforcer`; only use `DefaultRoleManager` and `DefaultDetector`. This can speed up test execution and reduce coupling.

**Constraints:**

* Integration modifications must keep the existing public API unchanged. Inject `detector` only via a setter; do not force all users to enable detection.
* Unit tests must use **JUnit 5** and ensure they pass within the project’s existing test suite.