Fix extensions field comment to use correct SEP-2133 terminology

Use "extension identifiers" (vendor-prefix/extension-name) instead
of "URIs" per the SEP-2133 Extensions specification.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: chughtapan

packages/core/src/types/types.ts

@@ -606,7 +606,7 @@ export const ServerCapabilitiesSchema = z.object({
      */
     tasks: ServerTasksCapabilitySchema.optional(),
     /**
-     * Extensions that the server supports. Keys are extension URIs.
+     * Extensions that the server supports. Keys are extension identifiers (vendor-prefix/extension-name).
      */
     extensions: z.record(z.string(), AssertObjectSchema).optional()
 });