docs(client-sdk): the error-handling example tells the truth — VALIDATION_FAILED, and category/retryable are optional

The example printed 'VALIDATION_ERROR' where data.create's validation
failure actually emits VALIDATION_FAILED (the registered extension code;
the batch-1 rename re-spelled the example's stale value without fixing
it), and showed category/retryable as populated when no server surface
emits either field today — they are optional contract fields the SDK now
reads from the declared spot (#4006), set only when the server sends them.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018u8vDQLWPJxBWQESznDpSj

Author: claude

content/docs/api/client-sdk.mdx

@@ -511,10 +511,10 @@ All errors follow a standardized format:
 try {
   await client.data.create('todo_task', { subject: '' });
 } catch (error) {
-  console.error(error.code);       // 'VALIDATION_ERROR'
-  console.error(error.category);   // 'validation'
+  console.error(error.code);       // 'VALIDATION_FAILED'
   console.error(error.httpStatus); // 400
-  console.error(error.retryable);  // false
+  console.error(error.category);   // optional — set only when the server sent it
+  console.error(error.retryable);  // optional — set only when the server sent it
   console.error(error.details);    // { ... }
 }
 ```