fix: use unknown cast for task result with looseObject index signature

The Result type now uses z.looseObject() which includes an index signature
[x: string]: unknown. TypeScript doesn't preserve this index signature
through object spreads, causing a type error when casting to SendResultT.

Use `as unknown as SendResultT` pattern (same as line 535) to bypass
the stricter type checking while maintaining runtime safety.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: ochafik

src/shared/protocol.ts

@@ -474,7 +474,7 @@ export abstract class Protocol<SendRequestT extends Request, SendNotificationT e
                                     taskId: taskId
                                 }
                             }
-                        } as SendResultT;
+                        } as unknown as SendResultT;
                     }
 
                     return await handleTaskResult();