Add Mistral AI provider support

Implements Mistral AI as an OpenAI-compatible provider with minimal customizations:

- Extends OpenAI provider for core functionality
- Custom Chat module to handle system role mapping (uses 'system' instead of 'developer')
- Custom render_payload to remove unsupported stream_options parameter
- Custom Embeddings module that ignores dimensions parameter (not supported by Mistral)
- Implements capabilities detection for vision (pixtral), embeddings, and chat models
- Adds ministral-3b-latest for chat tests (cheapest option)
- Adds pixtral-12b-latest for vision tests
- Adds mistral-embed for embedding tests
- Fetches and includes 63 Mistral models in models.json
- Adds appropriate test skips for known model limitations

Author: crmne

docs/available-models.md

@@ -20,6 +20,7 @@
   'bedrock' => 'Bedrock',
   'openrouter' => 'OpenRouter',
   'gpustack' => 'GPUStack',
+  'mistral' => 'Mistral',
   'pdf' => 'PDF'
 )
 loader.ignore("#{__dir__}/tasks")
@@ -79,15 +80,16 @@ def logger
   end
 end
 
-RubyLLM::Provider.register :openai, RubyLLM::Providers::OpenAI
 RubyLLM::Provider.register :anthropic, RubyLLM::Providers::Anthropic
-RubyLLM::Provider.register :gemini, RubyLLM::Providers::Gemini
-RubyLLM::Provider.register :deepseek, RubyLLM::Providers::DeepSeek
-RubyLLM::Provider.register :perplexity, RubyLLM::Providers::Perplexity
 RubyLLM::Provider.register :bedrock, RubyLLM::Providers::Bedrock
-RubyLLM::Provider.register :openrouter, RubyLLM::Providers::OpenRouter
-RubyLLM::Provider.register :ollama, RubyLLM::Providers::Ollama
+RubyLLM::Provider.register :deepseek, RubyLLM::Providers::DeepSeek
+RubyLLM::Provider.register :gemini, RubyLLM::Providers::Gemini
 RubyLLM::Provider.register :gpustack, RubyLLM::Providers::GPUStack
+RubyLLM::Provider.register :mistral, RubyLLM::Providers::Mistral
+RubyLLM::Provider.register :ollama, RubyLLM::Providers::Ollama
+RubyLLM::Provider.register :openai, RubyLLM::Providers::OpenAI
+RubyLLM::Provider.register :openrouter, RubyLLM::Providers::OpenRouter
+RubyLLM::Provider.register :perplexity, RubyLLM::Providers::Perplexity
 
 if defined?(Rails::Railtie)
   require 'ruby_llm/railtie'

lib/ruby_llm.rb

@@ -27,6 +27,7 @@ class Configuration
                   :ollama_api_base,
                   :gpustack_api_base,
                   :gpustack_api_key,
+                  :mistral_api_key,
                   # Default models
                   :default_model,
                   :default_embedding_model,