Add VectorSet tests when Redis 8.0 Docker image is available

[imperugo]

Problem

VectorSet operations (VADD, VSIM, VREM, etc.) require Redis 8.0+. The official `redis:latest` Docker image currently ships Redis 7.x, so we cannot run VectorSet tests in CI.

What's needed

1. Wait for `redis:8` or `redis:latest` to ship Redis 8.0
2. Add `CacheClientTestBase.VectorSet.cs` with tests covering:
   • Add vector + retrieve position
   • Similarity search returns ranked results
   • Remove member
   • Contains check
   • Length and dimension queries
   • Attributes JSON get/set
   • GetApproximateVector round-trip
   • GetLinks / GetLinksWithScores
3. Update CI workflow to use Redis 8.0+ image (or add a separate job)

Workaround

Tests can be run locally against a Redis 8.0 instance:
```bash
docker run -d --name redis8-test -p 6379:6379 redis:8.0-rc
dotnet test --filter "FullyQualifiedName~VectorSet"
```

Related

• PR Add VectorSet API for AI/ML similarity search #647 — VectorSet API implementation (merged without tests)
• Issue Add VectorSet API for AI/ML similarity search #631 — VectorSet feature request