Fix PermissionRequestResultKind type in custom-agents docs

Use PermissionRequestResultKind.Approved instead of string literal
"approved" to match the strongly-typed struct in the .NET SDK.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

docs/guides/custom-agents.md

@@ -158,7 +158,7 @@ await using var session = await client.CreateSessionAsync(new SessionConfig
         },
     },
     OnPermissionRequest = (req, inv) =>
-        Task.FromResult(new PermissionRequestResult { Kind = "approved" }),
+        Task.FromResult(new PermissionRequestResult { Kind = PermissionRequestResultKind.Approved }),
 });
 ```