Add Vertex gRPC and duration-bounded benchmarks

[kaizer113]

Summary

• add Vertex AI Vector Search query transports for REST, public gRPC, and private PSC gRPC
• add duration-bounded unrestricted closed-loop search for Redis and Vertex
• add reusable-resource safeguards, configurable Redis index names, synchronized worker warm-up, and Vertex upload quota retries

Why

The existing REST-only client included public-network and HTTP overhead in Vertex measurements and the harness could only run a fixed query count or fixed-rate open-loop workload. These changes let benchmarks use Vertex's native private gRPC path and measure maximum sustainable throughput for a fixed duration while preserving preloaded indexes between runs.

Impact

Set VERTEX_QUERY_TRANSPORT=private-grpc with VERTEX_GRPC_ADDRESS to benchmark a PSC/VPC endpoint, or use public-grpc for a public endpoint. Passing --search-duration without --target-qps now cycles queries in unrestricted closed loop for Redis and Vertex; --keep-data prevents cleanup and REDIS_INDEX_NAME selects an existing Redis index.

Vertex uploads now retry HTTP 429 quota responses, and timed workers establish their connections before the shared measurement clock begins.

Dependency

This is a follow-up to #146 and currently includes that PR's commit. Once #146 merges, GitHub will reduce this PR to the single follow-up commit automatically.

Validation

• make check
• make vector-db-benchmark
• make test — 79 passed
• make integration-test — 24 passed
• five-minute Redis and Vertex Glove-100 benchmark runs using the new duration and private gRPC paths

[fcostaoliveira]

Superseded by #148 (merged as 8303f1d), which consolidates this PR's code and applies the fairness/correctness fixes from a 24-agent adversarial review of the #145/#146/#147 stack (Redis prime+barrier symmetry, timed-window hoist, client_saturated fix, num_candidates clamp, overload-shed reporting, etc.). Closing as its functionality is now on master.