Add resolve method delegation from Models instance to class

The migration template uses RubyLLM.models.resolve which requires
the instance to delegate to the class method. This commit adds the
missing delegation and comprehensive tests using TDD approach.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: kieranklaassen

lib/ruby_llm/models.rb

@@ -217,6 +217,10 @@ def refresh!(remote_only: false)
       self.class.refresh!(remote_only: remote_only)
     end
 
+    def resolve(model_id, provider: nil, assume_exists: false, config: nil)
+      self.class.resolve(model_id, provider: provider, assume_exists: assume_exists, config: config)
+    end
+
     private
 
     def find_with_provider(model_id, provider)

spec/ruby_llm/models_spec.rb

@@ -143,6 +143,51 @@
     end
   end
 
+  describe '#resolve' do
+    it 'delegates to the class method when called on instance' do
+      # This tests that RubyLLM.models.resolve works
+      # which is needed in the migration template
+      model_id = 'gpt-4o'
+      provider = 'openai'
+      
+      # Call resolve on the instance (RubyLLM.models returns an instance)
+      model_info, provider_instance = RubyLLM.models.resolve(model_id, provider: provider)
+      
+      expect(model_info).to be_a(RubyLLM::Model::Info)
+      expect(model_info.id).to eq(model_id)
+      expect(model_info.provider).to eq(provider)
+      expect(provider_instance).to be_a(RubyLLM::Provider)
+    end
+
+    it 'resolves model without provider' do
+      model_id = 'gpt-4o'
+      
+      # Should resolve based on model ID alone
+      model_info, provider_instance = RubyLLM.models.resolve(model_id)
+      
+      expect(model_info).to be_a(RubyLLM::Model::Info)
+      expect(model_info.id).to eq(model_id)
+      expect(provider_instance).to be_a(RubyLLM::Provider)
+    end
+
+    it 'resolves with assume_exists option' do
+      model_id = 'custom-model'
+      provider = 'openai'
+      
+      # When assume_exists is true, it should create a default model
+      model_info, provider_instance = RubyLLM.models.resolve(
+        model_id, 
+        provider: provider, 
+        assume_exists: true
+      )
+      
+      expect(model_info).to be_a(RubyLLM::Model::Info)
+      expect(model_info.id).to eq(model_id)
+      expect(model_info.provider).to eq(provider)
+      expect(provider_instance).to be_a(RubyLLM::Provider)
+    end
+  end
+
   describe '#save_to_json' do
     it 'saves models to the models.json file' do
       temp_file = Tempfile.new(['models', '.json'])