fix: verify second listener called in test code

[mag123c]

Test code verify that the second listener was actually called

Fixed #47

Changes

Before:
The test "should not be affected by listener errors" in test/entity.spec.ts has an unused variable that Biome flagged with underscore prefix (_successCallCount).

After:
Test code verify that the second listener was actually called

expect(() => {
  user.updateProfile({ bio: "New bio" });
}).toThrow("Listener error");

expect(user.state.bio).toBe("New bio");
// Add assertion to verify second listener execution
expect(successCallCount).toBe(1);

[mag123c]

In the direction of removing subscription(), close this PR.