Add unit tests for models and services

- Implemented unit tests for the Package model, covering methods such as generateId, findById, findByName, and version management.
- Created unit tests for the Repository model, including repository creation, name validation, and retrieval methods.
- Added tests for the Session model, focusing on session creation, validation, and invalidation.
- Developed unit tests for the User model, ensuring user creation, password hashing, and retrieval methods function correctly.
- Implemented AuthService tests, validating login, token refresh, and session management.
- Added TokenService tests, covering token creation, validation, and revocation processes.

test/fixtures/pypi/demo_package/README.md

@@ -0,0 +1,11 @@
+# stack-test-demo-package
+
+Demo package for Stack.Gallery Registry e2e tests.
+
+## Usage
+
+```python
+from demo_package import greet
+
+print(greet("World"))  # Hello, World!
+```