Providers are disabled unless specifically configured

This is so that doing `models.refresh!` won't attempt to use every
provider API, resulting in errors unless a valid key is given for every one.

If this is too disruptive to be default behavior, then this could be a
"default to offline" mode that needs to be explicitly turned on perhaps
with an environment variable; in its absence, behavior would be the same
as before.

This is all so that `models.refresh!` can be called freely to populate
Ollama models at runtime.

Author: ldmosquera

lib/ruby_llm/provider.rb

@@ -29,6 +29,8 @@ def complete(messages, tools:, temperature:, model:, &block) # rubocop:disable M
       end
 
       def list_models
+        return [] unless enabled?
+
         response = connection.get(models_url) do |req|
           req.headers.merge! headers
         end

lib/ruby_llm/providers/anthropic.rb

@@ -15,6 +15,10 @@ module Anthropic
 
       module_function
 
+      def enabled?
+        !! RubyLLM.config.anthropic_api_key
+      end
+
       def api_base
         'https://api.anthropic.com'
       end

lib/ruby_llm/providers/deepseek.rb

@@ -8,6 +8,10 @@ module DeepSeek
 
       module_function
 
+      def enabled?
+        !! RubyLLM.config.deepseek_api_key
+      end
+
       def api_base
         'https://api.deepseek.com'
       end

lib/ruby_llm/providers/gemini.rb

@@ -15,6 +15,10 @@ module Gemini
 
       module_function
 
+      def enabled?
+        !! RubyLLM.config.gemini_api_key
+      end
+
       def api_base
         'https://generativelanguage.googleapis.com/v1beta'
       end

lib/ruby_llm/providers/openai.rb

@@ -28,6 +28,10 @@ def self.extended(base)
 
       module_function
 
+      def enabled?
+        !! RubyLLM.config.openai_api_key
+      end
+
       def api_base
         'https://api.openai.com/v1'
       end