feat: add credit_multiplier_for to fetch the model multiplier

lib/llm/models.rb

@@ -19,6 +19,10 @@ module Llm::Models
       models_for(feature).include?(model_name.to_s)
     end
 
+    def credit_multiplier_for(model_name)
+      CONFIG.dig('models', model_name.to_s, 'credit_multiplier') || 1
+    end
+
     def feature_config(feature_key)
       feature = features[feature_key.to_s]
       return nil unless feature

spec/lib/llm/models_spec.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Llm::Models do
+  describe '.credit_multiplier_for' do
+    it 'returns the credit multiplier for a known model' do
+      expect(described_class.credit_multiplier_for('gpt-4.1')).to eq(3)
+      expect(described_class.credit_multiplier_for('gpt-4.1-mini')).to eq(1)
+      expect(described_class.credit_multiplier_for('gpt-5.1')).to eq(2)
+    end
+
+    it 'returns 1 for an unknown model' do
+      expect(described_class.credit_multiplier_for('unknown-model')).to eq(1)
+    end
+
+    it 'returns 1 for nil' do
+      expect(described_class.credit_multiplier_for(nil)).to eq(1)
+    end
+  end
+end