Use webmock for API response mocking

[VoxSecundus]

No description provided.

[Copilot]

Pull Request Overview

This PR modernizes the test suite by replacing custom mock classes with WebMock for HTTP response mocking. The change improves test reliability by using real HTTP stubs instead of inheritance-based mocks.

Key changes:

• Implements WebMock-based HTTP request stubbing with comprehensive helper methods
• Replaces complex mock class inheritance patterns with clean stub configurations
• Expands documentation with detailed usage examples and configuration options

Reviewed Changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/test_helper.rb	Adds WebMock configuration and comprehensive helper methods for stubbing API responses
test/test_user_client.rb	Replaces mock class inheritance with WebMock stubs for cleaner test setup
test/test_listen_client.rb	Converts from mock classes to WebMock stubs with parameterized request handling
README.md	Enhances documentation with detailed usage examples and configuration instructions
Gemfile	Adds webmock dependency in test group
.junie/guidelines.md	Updates development guidelines to mandate WebMock usage

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