Add RubyLLM::Test to Ecosystem

docs/_reference/ecosystem.md

@@ -30,6 +30,7 @@ After reading this guide, you will know:
 * How `RubyLLM::Monitoring` provides dashboards and alerts for RubyLLM activity
 * How `RubyLLM::RedCandle` enables local model execution from Ruby
 * How OpenTelemetry instrumentation for RubyLLM provides observability into your LLM applications
+* How to test application code by stubbing responses with `RubyLLM::Test`
 * Where to find community projects and how to contribute your own
 
 ## RubyLLM::Schema
@@ -282,6 +283,48 @@ For detailed documentation and examples, visit the [RubyLLM::Tribunal repository
 
 ---
 
+## RubyLLM::Test
+
+**Test Application Code by Stubbing LLM Responses**
+
+[`RubyLLM::Test`](https://github.com/RockSolt/ruby_llm-test) allows you to stub LLM responses in your tests, making it easier to test application logic without relying on calls to external systems.
+
+### Why Use RubyLLM::Test?
+
+When writing tests for code that interacts with LLMs, you may want to:
+
+- Ensure your application logic behaves correctly without making real API calls
+- Test edge cases and error handling
+- Control the responses from the LLM for deterministic tests
+
+### Key Features
+
+- Clear syntax for defining stubs and expected responses
+- Support for multiple stubs in a single test
+- Validate arguments, such as model or tool calls, passed to the LLM
+- Works with RSpec and Minitest
+
+### Usage
+
+```ruby
+RubyLLM::Test.stub_response("Outlook good")
+
+chat = RubyLLM.chat
+response = chat.ask "What are the odds this works?"
+
+assert_equal "Outlook good", response.content
+```
+
+### Installation
+
+Add the gem to the test group in your Gemfile, or install it directly:
+
+```bash
+gem install ruby_llm-test
+```
+
+---
+
 ## Community Projects
 
 The RubyLLM ecosystem is growing! If you've built a library or tool that extends RubyLLM, we'd love to hear about it. Consider: