vecindex: convert vecstore for use with non-transactional fixups

Previously, the vecstore was not compatible with the new non-transactional
split operation, because it did not implement the upgraded cspann.Store
interface. This commit completes that implementation, including the new
Try methods, as well as making some changes to the semantics of the
methods on cspann.Txn.

Epic: CRDB-42943
Release note: None

Author: andy-kimball

pkg/sql/vecindex/cspann/commontest/BUILD.bazel

@@ -13,7 +13,6 @@ go_library(
         "//pkg/sql/vecindex/cspann/quantize",
         "//pkg/sql/vecindex/cspann/testutils",
         "//pkg/sql/vecindex/cspann/workspace",
-        "//pkg/util/num32",
         "//pkg/util/vector",
         "@com_github_cockroachdb_errors//:errors",
         "@com_github_stretchr_testify//require",