Correct VLINKS return types

[hutiefang76]

Fixes #3404

VLINKS returns one collection per HNSW graph layer. The existing outputs flatten those layers into a single list or map, which also drops empty layers.

This change:

• exposes List<List<V>> and List<Map<V, Double>> through the sync, async, cluster, and coroutine APIs
• emits one item per graph layer in reactive APIs (Flux<List<V>> and Flux<Map<V, Double>>)
• adds typed outputs that preserve graph-layer boundaries for RESP2 arrays and RESP3 maps
• covers empty layers, missing keys, wrong key types, and invalid score responses

Tests:

• RedisVectorSetCommandBuilderUnitTests: 38 passed on JDK 8
• RedisVectorSetAdvancedIntegrationTests, RedisVectorSetResp2IntegrationTests, and RedisVectorSetIntegrationTests: 87 passed against Redis 8.6.2 on JDK 17
• formatter validation and six API generator suites passed

---

Note

Medium Risk
Breaking API change on experimental vector-set commands; behavior change is intentional correctness fix with broad surface area (sync/async/reactive/cluster/Kotlin).

Overview
Fixes incorrect modeling of Redis VLINKS responses: the server returns one collection per HNSW graph layer, but Lettuce previously flattened layers into a single List or Map and could drop empty layers.

vlinks and vlinksWithScores now expose List<List<V>> and List<Map<V, Double>> across sync, async, reactive, cluster, coroutines, and API templates. Reactive APIs emit Flux<List<V>> / Flux<Map<V, Double>> (one element per layer) instead of dissolving into flat streams or a single Mono.

Command decoding switches from ValueListOutput / ValueDoubleMapOutput to new ValueListListOutput and ValueDoubleMapListOutput, preserving layer boundaries for RESP2 arrays and RESP3 maps. Unit tests cover layered decode, RESP3 maps, and invalid scores; integration tests were updated for the new shapes (including merging score maps per layer where assertions need flat neighbor scores).

Breaking change for callers of these @Experimental methods who assumed a flat list or map.

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[a-TODO-rov]

Thanks @hutiefang76
Will review this soon