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/demo-gem.gemspec

@@ -0,0 +1,16 @@
+Gem::Specification.new do |spec|
+  spec.name          = "stack-test-demo-gem"
+  spec.version       = "1.0.0"
+  spec.authors       = ["Stack.Gallery Test"]
+  spec.email         = ["test@stack.gallery"]
+
+  spec.summary       = "Demo gem for Stack.Gallery Registry e2e tests"
+  spec.description   = "A demonstration gem for testing Stack.Gallery Registry"
+  spec.homepage      = "https://github.com/stack-gallery/demo-gem"
+  spec.license       = "MIT"
+
+  spec.required_ruby_version = ">= 2.7.0"
+
+  spec.files         = Dir["lib/**/*", "README.md"]
+  spec.require_paths = ["lib"]
+end