cspann: reduce unneccessary fixups

[Lloyd-Pottiger]

• Added SearchOptions.DisableSplitMergeFixups (pkg/sql/vecindex/cspann/index.go:117), whereby the searcher skips ‘size-based’ split/merge fixups when this option is enabled enqueue operations (stalled-op fixups are still enqueued) (pkg/sql/vecindex/cspann/searcher.go:453).
• The internal search for reassignFromSameLevel / startMerge enables this option under non-deterministic indexes, preventing the enqueuing of potentially stale fixups before subsequent TryMoveVector operations alter partition sizes enqueue a batch of potentially stale fixups before TryMoveVector alters partition size (pkg/sql/vecindex/cspann/fixup_split.go:539, pkg/sql/vecindex/cspann/fixup_merge.go:145).
• Added test pkg/sql/vecindex/cspann/searcher_fixups_test.go and included Bazel target (pkg/sql/vecindex/cspann/BUILD.bazel:55); bazel test //pkg/sql/vecindex/cspann:cspann_test has been run.

Epic: None
Release: None

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[cockroach-teamcity]

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[blathers-crl]

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