Address PR feedback: add structuredContent assertion and changeset

Add structuredContent assertion to callToolStream test for parity with
callTool test, and add changeset file for the patch.

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

Author: joaquinhui

.changeset/skip-structured-content-validation-on-error.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/sdk": patch
+---
+
+Skip structuredContent validation when tool result has isError: true

test/integration/test/client/client.test.ts

@@ -4161,6 +4161,7 @@ test('callToolStream() should not validate structuredContent when isError is tru
     if (messages[0]!.type === 'result') {
         expect(messages[0]!.result.isError).toBe(true);
         expect(messages[0]!.result.content).toEqual([{ type: 'text', text: 'Something went wrong' }]);
+        expect(messages[0]!.result.structuredContent).toEqual({ wrongField: 123 });
     }
 
     await client.close();