✨ feat: add DEFAULT_MODEL environment variable (#280)

* ✨ feat: add DEFAULT_MODEL environment variable

* set the model maxLength setting in the models definition

* set the model tokenLimit setting in the models definition

types/openai.ts

@@ -1,6 +1,8 @@
 export interface OpenAIModel {
   id: string;
   name: string;
+  maxLength: number; // maximum length of a message
+  tokenLimit: number;
 }
 
 export enum OpenAIModelID {
@@ -8,13 +10,20 @@ export enum OpenAIModelID {
   GPT_4 = 'gpt-4',
 }
 
+// in case the `DEFAULT_MODEL` environment variable is not set or set to an unsupported model
+export const fallbackModelID = OpenAIModelID.GPT_3_5;
+
 export const OpenAIModels: Record<OpenAIModelID, OpenAIModel> = {
   [OpenAIModelID.GPT_3_5]: {
     id: OpenAIModelID.GPT_3_5,
-    name: 'Default (GPT-3.5)',
+    name: 'GPT-3.5',
+    maxLength: 12000,
+    tokenLimit: 3000,
   },
   [OpenAIModelID.GPT_4]: {
     id: OpenAIModelID.GPT_4,
     name: 'GPT-4',
+    maxLength: 24000,
+    tokenLimit: 6000,
   },
 };