refactor: Make AIConfig inherit from AIConfigDefault

AIConfigDefault serves as the base class holding the shared attr_readers
(enabled, messages, model, provider) and to_h. AIConfig extends it with
tracker_factory and create_tracker only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jsonbailey

lib/server/ai/client.rb

@@ -140,15 +140,10 @@ def to_h
       # a {#create_tracker} factory. Do not instantiate directly — use
       # {AIConfigDefault} for fallback values.
       #
-      class AIConfig
-        attr_reader :enabled, :messages, :model, :provider
-
+      class AIConfig < AIConfigDefault
         def initialize(tracker_factory:, enabled: nil, model: nil, messages: nil, provider: nil)
-          @enabled = enabled
-          @messages = messages
+          super(enabled: enabled, model: model, messages: messages, provider: provider)
           @tracker_factory = tracker_factory
-          @model = model
-          @provider = provider
         end
 
         #
@@ -159,17 +154,6 @@ def initialize(tracker_factory:, enabled: nil, model: nil, messages: nil, provid
         def create_tracker
           @tracker_factory.call
         end
-
-        def to_h
-          {
-            _ldMeta: {
-              enabled: @enabled || false,
-            },
-            messages: @messages.is_a?(Array) ? @messages.map { |msg| msg&.to_h } : nil,
-            model: @model&.to_h,
-            provider: @provider&.to_h,
-          }
-        end
       end
 
       #