chore(fai): Update param types. (#574)

Author: eyw520

fern/apis/fai/definition/chat.yml

@@ -19,19 +19,32 @@ service:
         name: ChatCompletionRequest
         body:
           properties:
-            model: optional<string>
-            system_prompt: optional<string>
-            messages: list<ChatTurn>
+            model:
+              type: LanguageModel
+              docs: The model to use for the chat completion.
+            system_prompt:
+              type: optional<string>
+              docs: The system prompt to use for the chat completion.
+            messages: 
+              type: list<ChatTurn>
+              docs: The messages to use for the chat completion.
       response: ChatCompletionResponse
       errors:
         - commons.BadRequestError
         - commons.InternalError
 
 types:
   ChatCompletionResponse:
-    type: list<ChatTurn>
+    properties:
+      turns: list<ChatTurn>
+      citations: list<string>
 
   ChatTurn:
     properties:
       role: string
       content: string
+
+  LanguageModel:
+    enum:
+      - claude-4-sonnet-20250514
+      - command-a-03-2025