fix: normalize null to undefined in ElicitResultSchema content field (#1204)

Author: mattzcarey

src/types.ts

@@ -1831,8 +1831,13 @@ export const ElicitResultSchema = ResultSchema.extend({
     /**
      * The submitted form data, only present when action is "accept".
      * Contains values matching the requested schema.
+     * Per MCP spec, content is "typically omitted" for decline/cancel actions.
+     * We normalize null to undefined for leniency while maintaining type compatibility.
      */
-    content: z.record(z.string(), z.union([z.string(), z.number(), z.boolean(), z.array(z.string())])).optional()
+    content: z.preprocess(
+        val => (val === null ? undefined : val),
+        z.record(z.string(), z.union([z.string(), z.number(), z.boolean(), z.array(z.string())])).optional()
+    )
 });
 
 /* Autocomplete */