fix(uts/objects): align REST fixture provisioning with V2 objects API

[sacOO7]

Summary

The provision_objects_via_rest REST fixture helper in uts/objects/helpers/standard_test_pool.md
described the legacy / pre-V2 objects REST shape and posted to the legacy prod-sandbox host. This
aligns it (and its only call site) with the V2 contract (per the LiveObjects OpenAPI specification,
ably-docs/static/open-specs/liveobjects.yaml, "Update LiveObjects REST API docs for V2 format",
2026-01-22) and with the canonical nonprod sandbox host every other UTS integration spec uses.

What changed

1. V2 request shape

uts/objects/helpers/standard_test_pool.md — the provision_objects_via_rest helper:

	Before (pre-V2)	After (V2)
Endpoint	POST …/channels/{ch}/objects (plural)	POST …/channels/{ch}/object (singular)
Body envelope	{ "messages": [ … ] }	a single operation or a bare JSON array (batch) — no envelope
Operation encoding	nested operation: { action: "MAP_SET", objectId, mapSet:{…} }	payload-key discriminator mapSet:{…} + sibling objectId/path
Target	objectId only	objectId or path
Extras	—	optional idempotency id; string/bytes may carry encoding

It now documents each operation shape (mapSet, mapRemove, mapCreate, counterInc, counterCreate),
the { string | number | boolean | bytes | objectId } primitive values, and mapCreate.semantics as the
integer 0 (LWW) with { data: … }-wrapped entries.

uts/objects/integration/objects_lifecycle_test.md — the only caller (RTPO15,
rest-provisioned-data-sync-0) updated to the V2 operation shape, so the spec stays internally consistent:

-  {
-    operation: {
-      action: "MAP_SET",
-      objectId: "root",
-      mapSet: { key: "provisioned", value: { string: "from_rest" } }
-    }
-  }
+  {
+    mapSet: { key: "provisioned", value: { string: "from_rest" } },
+    objectId: "root"
+  }

2. Sandbox host

uts/objects/helpers/standard_test_pool.md — the provision_objects_via_rest POST URL:

-  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

sandbox-rest.ably.io was a stale leftover: the objects integration specs were migrated to the nonprod
host in e57d340e, but the pooled helper was missed — leaving it provisioning fixtures on a different
backend than the app/key are created on, so RTPO15 could never resolve. sandbox.realtime.ably-nonprod.net
is the host the entire UTS integration corpus uses — raw HTTP (app provisioning) uses the explicit URL, and
SDK clients use endpoint: "nonprod:sandbox", which resolves to the same host. sandbox-rest.ably.io now
appears nowhere in the objects module.

Validation

Every element was checked field-by-field against the V2 OpenAPI: singular /object path, OperationRequest
= single-or-array (no messages), mapSet = {key, value: PrimitiveValue}, mapCreate.semantics integer
0, objectId/path targets, idempotency id, and the PrimitiveValue set (incl. optional encoding).
The host was cross-checked against every realtime/integration and rest/integration spec: 82 raw-HTTP
references to https://sandbox.realtime.ably-nonprod.net and 196 endpoint: "nonprod:sandbox" client
configs; sandbox-rest.ably.io is used by no other spec file.

Context

Surfaced while deriving the ably-java integration helper
(uts/.../integration/standard/liveobjects/helpers.kt in ably-java), which is built to this same V2 shape
and the same nonprod host (via restHost = ProxyManager.sandboxRealtimeHost).

3. Sandbox host on the integration clients

uts/objects/integration/objects_lifecycle_test.md (10 client blocks) and objects_sync_test.md (5) set
no host on their Realtime(...) options:

- Realtime(options: { key: api_key, useBinaryProtocol: PROTOCOL == "msgpack" })
+ Realtime(options: { key: api_key, endpoint: "nonprod:sandbox", useBinaryProtocol: PROTOCOL == "msgpack" })

Without an endpoint the SDK falls back to the production host (realtime.ably.io, REC1a), so the clients
would never reach the nonprod sandbox the "Sandbox Setup" provisions the app on — the header was
aspirational. Every other UTS integration spec configures clients with endpoint: "nonprod:sandbox"; this
adds it to match. (The proxy spec objects_faults.md already routes correctly — nonprod:sandbox target +
localhost proxy hop.)