Make FunctionDeclaration description required

Author: hsubox76

packages/vertexai/src/types/requests.ts

@@ -145,10 +145,10 @@ export declare interface FunctionDeclaration {
    */
   name: string;
   /**
-   * Optional. Description and purpose of the function. Model uses it to decide
+   * Description and purpose of the function. Model uses it to decide
    * how and whether to call the function.
    */
-  description?: string;
+  description: string;
   /**
    * Optional. Describes the parameters to this function in JSON Schema Object
    * format. Reflects the Open API 3.03 Parameter Object. Parameter names are