fix(uts): align provision_objects_via_rest host with the nonprod sandbox

The REST fixture-provisioning helper still POSTed to https://sandbox-rest.ably.io,
the legacy prod-sandbox host. Every other objects integration spec — and the whole
UTS integration corpus (realtime + rest) — provisions apps and routes clients via
https://sandbox.realtime.ably-nonprod.net (raw HTTP) / endpoint: "nonprod:sandbox"
(SDK clients). The objects integration specs were migrated to the nonprod host in
e57d340, but standard_test_pool.md was missed, leaving provision_objects_via_rest
posting fixtures to a different backend than the app/key were provisioned on — so
RTPO15 (rest-provisioned-data-sync) could never resolve.

Point the helper at the canonical nonprod host to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: sacOO7

uts/objects/helpers/standard_test_pool.md

@@ -363,7 +363,7 @@ The objects REST API uses the **V2 format** (per the LiveObjects OpenAPI specifi
 ```pseudo
 provision_objects_via_rest(api_key, channel_name, operations):
   # operations: a single operation object, or an array of operation objects (batch)
-  POST https://sandbox-rest.ably.io/channels/{encode_uri_component(channel_name)}/object
+  POST https://sandbox.realtime.ably-nonprod.net/channels/{encode_uri_component(channel_name)}/object
     WITH Authorization: Basic {base64(api_key)}
     WITH Content-Type: application/json
     WITH body: operations