chore(internal): improve index signature formatting (#739)

Author: stainless-app[bot]

src/resources/beta/messages/messages.ts

@@ -678,6 +678,7 @@ export namespace BetaTool {
     type: 'object';
 
     properties?: unknown | null;
+
     [k: string]: unknown;
   }
 }

src/resources/messages/messages.ts

@@ -672,6 +672,7 @@ export namespace Tool {
     type: 'object';
 
     properties?: unknown | null;
+
     [k: string]: unknown;
   }
 }