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/README.md

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+# stack-test-demo-gem
+
+Demo gem for Stack.Gallery Registry e2e tests.
+
+## Usage
+
+```ruby
+require 'demo-gem'
+
+puts StackTestDemoGem.greet("World") # Hello, World!
+```

test/fixtures/rubygems/demo-gem/demo-gem.gemspec

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+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

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

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+# 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