Read modelKey/modelVersion from _ldMeta, matching gonfalon PR 67230

Gonfalon moved these two fields from the payload's model object to
_ldMeta (launchdarkly/gonfalon#67230) to avoid modelVersion reading as
the underlying LLM's own version. Field names are unchanged; only the
JSON location moves. ModelConfig's public accessors and the tracker's
stamped event fields are untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: atornsii

lib/server/ai/client.rb

@@ -285,16 +285,15 @@ def _completion_config(key:, context:, default:, variables: nil)
             provider_config = ProviderConfig.new(provider_config.fetch(:name, ''))
           end
 
-          tracked_model_version = 1
+          tracked_model_version = (variation.dig(:_ldMeta, :modelVersion) || 1).to_i
           if (model = variation[:model]) && model.is_a?(Hash)
             parameters = variation[:model][:parameters]
             custom = variation[:model][:custom]
-            tracked_model_version = (variation[:model][:modelVersion] || 1).to_i
             model = ModelConfig.new(
               name: variation[:model][:name],
               parameters: parameters,
               custom: custom,
-              model_key: variation[:model][:modelKey],
+              model_key: variation.dig(:_ldMeta, :modelKey),
               model_version: tracked_model_version
             )
           end

spec/server/ai/client_spec.rb

@@ -93,12 +93,10 @@
         {
           model: {
             name: 'gpt-4',
-            modelKey: 'my-model',
-            modelVersion: 2,
           },
           provider: { name: 'openai' },
           messages: [],
-          _ldMeta: { enabled: true, variationKey: 'v1', version: 1 },
+          _ldMeta: { enabled: true, variationKey: 'v1', version: 1, modelKey: 'my-model', modelVersion: 2 },
         }
       )
       .variation_for_all(0))
@@ -487,10 +485,10 @@
         td.update(td.flag('model-config-version-only')
           .variations(
             {
-              model: { name: 'gpt-4', modelVersion: 3 },
+              model: { name: 'gpt-4' },
               provider: { name: 'openai' },
               messages: [],
-              _ldMeta: { enabled: true, variationKey: 'v1', version: 1 },
+              _ldMeta: { enabled: true, variationKey: 'v1', version: 1, modelVersion: 3 },
             }
           )
           .variation_for_all(0))