feat: add completions capability in ServerCapabilitiesSchema

Author: morinokami

src/types.ts

@@ -247,6 +247,10 @@ export const ServerCapabilitiesSchema = z
      * Present if the server supports sending log messages to the client.
      */
     logging: z.optional(z.object({}).passthrough()),
+    /**
+     * Present if the server supports sending completions to the client.
+     */
+    completions: z.optional(z.object({}).passthrough()),
     /**
      * Present if the server offers any prompt templates.
      */