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/rubygems/demo-gem/lib/demo-gem.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+# Demo gem for Stack.Gallery Registry e2e tests
+module StackTestDemoGem
+  VERSION = "1.0.0"
+
+  # Greet the given name
+  # @param name [String] The name to greet
+  # @return [String] A greeting message
+  def self.greet(name)
+    "Hello, #{name}!"
+  end
+end