Closes #6171. Part of #2728.

What this does

Resolves the Couchbase scope and collection through a single _scope_and_collection
helper, so that with registry.enable_online_feature_view_versioning: true each feature
view version gets its own scope and collection
(test_project_driver_stats_v2_scope / ..._v2_collection) instead of all versions
sharing one. Applied to all four call sites — online_write_batch, online_read,
update, teardown.

Couchbase already namespaced by {project}_{table.name}, so this uses
compute_table_id, the same convention as the merged Milvus (#6330) and FAISS
(#6256) stores. With versioning disabled the names are byte-identical to today's, which
the versioning=False cases pin.

Two decisions worth calling out

Document ids are left unversioned. _document_id still returns
{project}:{table.name}:{entity_key}:{feature}. A Couchbase collection is already a
namespace, so partitioning at the collection level is sufficient to keep versions from
colliding, and leaving ids alone keeps them stable for existing data. Happy to version
them too if you'd rather have the version visible in the key.

Couchbase is added to the versioned-read allowlist in
OnlineStore._is_versioned_read_supported(). I checked the read path rather than
assuming: CouchbaseOnlineStore.online_read honours the OnlineStore contract as
sqlite.py defines it — one entry per requested entity key, in request order, holding
ValueProto values parsed from the stored payload, and (None, None) on
DocumentNotFoundException. So versioned reads genuinely work here once the collection
is version-scoped.

Drive-by: a stale error message

VersionedOnlineReadNotSupported listed "SQLite, PostgreSQL, MySQL, FAISS, Redis, and
DynamoDB" — but _is_versioned_read_supported() has included MilvusOnlineStore since
#6330, which updated online_store.py without touching errors.py. So the message has
been telling Milvus users their store is unsupported while the code supported it. Added
Milvus alongside Couchbase. Happy to split this into its own PR if you'd prefer.

Tests

New sdk/python/tests/unit/infra/online_store/test_couchbase_versioning.py, 11 tests,
MagicMock-based in the same style as the merged test_milvus_versioning.py, and
importorskip-guarded on couchbase.

• naming: unversioned unchanged; version ignored while the flag is off; _v2 when on;
  projection.version_tag takes priority over current_version_number; version 0 gets
  no suffix; two versions never collide
• routing: update connects with and creates the versioned scope, teardown drops the
  versioned scope — parametrized so the unversioned case is asserted as the control
• allowlist: _is_versioned_read_supported() is true, and _check_versioned_read_support
  no longer raises for a version-qualified ref

Verified red-before/green-after: with the three source files reverted, the four
behavioural tests fail and the versioning=False control still passes, so they test the
change rather than the setup.

Regression check across the 55 unit test files touching online stores or versioning: the
set of failing and erroring tests is identical with and without this change (diff of
the sorted FAILED/ERROR lines is empty). Those pre-existing failures are missing
optional dependencies in my environment. ruff check, ruff format and mypy are clean
on all four files.

🤖 Written with Claude Code (Claude Opus 5), reviewed by @arose26.