fix: allow explicit CuDNN with CUDA minor versions

[sarptandoven]

Summary

• Fix CUDA/CuDNN compatibility validation to treat user-specified CUDA minor versions like 11.6 as selectors for compatible patch-level base images such as 11.6.2.
• Add a regression test that completes a GPU config with explicit cuda: "11.6" and cudnn: "8", then verifies the selected NVIDIA base image.

Why

latestCuDNNForCUDA and cudaBaseImageFor already use version.Matches, so cuda: "11.6" can resolve to nvidia/cuda:11.6.2-cudnn8-devel-ubuntu20.04. The validation path used an exact string comparison, which meant the same config failed early when cudnn was set explicitly.

Test plan

• git diff --check
• go test ./pkg/config -run 'TestCUDABaseImageTag|TestLatestCuDNNForCUDA|TestValidateAndCompleteCUDA' -count=1
• go test ./pkg/config -count=1

[anish-sahoo]

lgtm