FEATURE: configure a default LLM model for all features

This update adds a setting to the configure a default LLM model to be used for all features (unless overridden).

Author: keegangeorge

config/locales/server.en.yml

@@ -25,6 +25,7 @@ en:
         description: "Periodic report based on a large language model"
   site_settings:
     discourse_ai_enabled: "Enable the discourse AI plugin."
+    ai_default_llm_model: "The default LLM model to use for all AI features"
     ai_artifact_security: "The AI artifact system generates IFRAMEs with runnable code. Strict mode forces an extra click to run code. Lax mode runs code immediately. Hybrid mode allows user to supply data-ai-artifact-autorun to show right away. Disabled mode disables the artifact system."
     ai_toxicity_enabled: "Enable the toxicity module."
     ai_toxicity_inference_service_api_endpoint: "URL where the API is running for the toxicity module"

config/settings.yml

@@ -11,6 +11,12 @@ discourse_ai:
       - "lax"
       - "hybrid"
       - "strict"
+  ai_default_llm_model:
+    default: ""
+    type: enum
+    allow_any: false
+    enum: "DiscourseAi::Configuration::LlmEnumerator"
+    validator: "DiscourseAi::Configuration::LlmValidator"
 
   ai_sentiment_enabled:
     default: false