Move the proto-free vector index test helpers into the core test fixtures

[normen662]

Moves the vector index test helpers that don't depend on a record proto from VectorIndexTestBase and GuardiannVectorIndexConcurrentMergeTest into a new VectorIndexTestSupport in the core test fixtures, so that they are published with them (see #4676) and can be reused with other record protos.

• Merging a vector index (once, or to completion) and checking it for outstanding work.
• Building a by-distance index plan for a query vector of any precision and dimensionality (the literal's type now comes from the query vector instead of a hard-coded VECTOR(128, HALF)).
• Inserting a batch with retries on conflicts and on VectorIndexClusterTooLargeException.
• A raw Guardiann view of a record-store vector index, the Guardiann structure checks, and recall@k.

VectorIndexTestBase (which stays in src/test, with its proto) and GuardiannVectorIndexConcurrentMergeTest delegate to it; their signatures and behavior are unchanged (the index plans built for the existing 128-dimensional half-precision callers are identical, including plan hashes and serialization). The core test fixtures now expose the fdb-extensions test fixtures as an API dependency.

Verified: the vector index and Guardiann tests of fdb-record-layer-core pass; VectorIndexTestSupport is part of the published test fixtures jar (it doesn't reference any proto); check passes.