fix(vectorsets): VLinks return type (#3820)

* fix(vset): vlinks return type

* fix(vset): fix test

* fix(vset): fix test

* fix(vset): remove last layer check from tests

* fix(vset): fix lint & issue

* fix(vset): fix n -> innerLen check

* test(vset): add mocked-response parser tests for VLINKS

Cover StringSliceSliceCmd.readReply (VLINKS) and
VectorScoreSliceSliceCmd.readReply (VLINKS WITHSCORES) with both
RESP2 and RESP3 wire fixtures, including empty layers, nil
elements, odd-length-array errors, and malformed input.

Also switch NewStringSliceSliceCmd to args ...any to match the
sibling NewVectorScoreSliceSliceCmd signature.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Nedyalko Dyakov <nedyalko.dyakov@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Nedyalko Dyakov <1547186+ndyakov@users.noreply.github.com>