test: adding 2 additional tests to user.py (#595)

# Description

Adding 2 more tests to user.py to improve build code coverage

- [X] Follow the [`CONTRIBUTING`
Guide](https://github.com/a2aproject/a2a-python/blob/main/CONTRIBUTING.md).
- [X] Make your Pull Request title in the
<https://www.conventionalcommits.org/> specification.
- [X] Ensure the tests and linter pass (Run `bash scripts/format.sh`
from the repository root to format)
- [N/A] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

N/A

---------

Co-authored-by: Lukasz Kawka <luk.kawka@gmail.com>

Author: didier-durand

tests/auth/test_user.py

@@ -1,9 +1,19 @@
 import unittest
 
-from a2a.auth.user import UnauthenticatedUser
+from inspect import isabstract
+
+from a2a.auth.user import UnauthenticatedUser, User
+
+
+class TestUser(unittest.TestCase):
+    def test_is_abstract(self):
+        self.assertTrue(isabstract(User))
 
 
 class TestUnauthenticatedUser(unittest.TestCase):
+    def test_is_user_subclass(self):
+        self.assertTrue(issubclass(UnauthenticatedUser, User))
+
     def test_is_authenticated_returns_false(self):
         user = UnauthenticatedUser()
         self.assertFalse(user.is_authenticated)