feat: expose cuVS CAGRA num_random_samplings search param

[devin-ai-integration]

Summary

Plumbs the cuVS cagra::search_params field num_random_samplings through Knowhere's GPU_CUVS CAGRA index so it can be set at search time from Milvus.

cuVS CAGRA seeds its beam search with num_random_samplings random dataset nodes (cuVS default 1). At large per-segment N, 1 gives poor recall; raising it (e.g. 8) recovers recall for ~30% latency overhead, with no reindex (it is a search-time param). Knowhere previously did not declare this field, so it stayed pinned at the cuVS default of 1.

The change is purely additive and mirrors the existing search-param plumbing (itopk_size, search_width, team_size, …) across the three layers:

# src/index/gpu_cuvs/gpu_cuvs_cagra_config.h  (user-facing config)
+ CFG_INT num_random_samplings;
+ KNOWHERE_CONFIG_DECLARE_FIELD(num_random_samplings)
+     .description("number of initial random seed node sampling iterations")
+     .set_default(1)
+     .set_range(1, std::numeric_limits<CFG_INT::value_type>::max())
+     .for_search();
+ result.num_random_samplings = cfg.num_random_samplings;   // in to_cuvs_knowhere_config()

# src/common/cuvs/integration/cuvs_knowhere_config.hpp  (cuVS-agnostic config)
+ std::optional<int> num_random_samplings = std::nullopt;
+ config.num_random_samplings = config.num_random_samplings.value_or(1);  // in validate_cuvs_knowhere_config()

# src/common/cuvs/integration/cuvs_knowhere_index.cuh  (map into cuvs cagra::search_params)
+ result.num_random_samplings = *(config.num_random_samplings);  // in config_to_search_params<cagra>()

num_random_samplings maps 1:1 to cuVS cagra::search_params::num_random_samplings (uint32_t, default 1, valid >= 1).

Backward compatibility & risk

Backward compatible: default 1 reproduces the current cuVS default exactly, so behavior is unchanged unless a user explicitly sets the param. Search-time only (.for_search()), so no reindex and no serialization/ABI impact. Total: 3 files, +10 / -0 LOC.

Link to Devin session: https://6sense.devinenterprise.com/sessions/ca163e56725d4f04978fd68f0162db5b

[devin-ai-integration]

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