fix(nvca): default NVMesh model cache volumes to nvcf-sc-30, not nvcf-sc

[apartha-nv]

Summary

• doModelCacheNVMesh only runs once SelectHelmCacheBackend has confirmed nvcf-sc-30 exists in the cluster, but the unconfigured storage-class default it applied to the PVC was DefaultModelCacheStorageClassName ("nvcf-sc") — the Samba-oriented default. Since both classes share the same NVMesh CSI provisioner (nvmesh-csi.excelero.com), volumes silently landed on the wrong tier instead of failing loudly.
• Adds nvmeshModelCacheStorageClassName, used only for NewReconciler's one-time resolution of modelCacheStorageClass (consumed exclusively by doModelCacheNVMesh), defaulting to NVMeshStorageClassName.
• Splits off a new sambaModelCacheStorageClass field, resolved from the same override via the original ModelCacheStorageClassName default, so doModelCacheSamba's backing PVC keeps landing on nvcf-sc regardless of the NVMesh-side default change.

Test plan

• TestNvmeshModelCacheStorageClassNameResolution — new unit test pinning the NVMesh default
• TestModelCacheStorageClassResolvedOnce — updated to expect nvcf-sc-30 as the NVMesh default
• TestSambaModelCacheStorageClassResolvedOnce — new unit test confirming Samba's field independently still defaults to nvcf-sc
• TestApplyModelCacheStorageClass — updated expectation
• TestReconcile_ModelCacheSamba (envtest) — extended to assert the Samba backing PVC's storage class end-to-end
• go test ./pkg/storage/... passes in full (envtest via KUBEBUILDER_ASSETS)
• go vet ./pkg/storage/... clean

Marked as draft — opening for early feedback before final review.

[coderabbitai]

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