Add unit tests for cmd/gpuop-cfg config validators

[abrarshivani]

Description

Adds hermetic unit tests for the cmd/gpuop-cfg/validate subtree (previously 0% covered). Tests only — no production source changed. No test reaches registry access: every image case fails during image-path construction or reference parsing before ManifestGet (control-flow, not just verified behind a dead proxy).

Covered

• csv.validateALMExample — 100%: first item Kind == ClusterPolicy → nil; malformed JSON → "invalid character"; missing/empty/null/nil-annotation → unmarshal or "no example clusterpolicy found"; JSON object instead of array → "cannot unmarshal object"; empty list → "no example clusterpolicy found"; first item Kind != ClusterPolicy (including when a ClusterPolicy appears later) → "invalid example clusterpolicy". (Note: the validator only checks cpList[0].Kind; the multi-entry cases document that contract.)
• options.load / options.getContents (csv and clusterpolicy) — 100%: valid manifest, missing file, malformed YAML, empty file (via t.TempDir), and the input == "-" stdin branch (via an os.Pipe redirect). Local file/stdin I/O is deterministic and hermetic.
• validateImage / validateImages (csv and clusterpolicy) — error paths only: an invalid image reference fails at ref.New before any registry call; an empty/zero spec fails at ImagePath before any registry access. Assertions use require.ErrorContains.
• NewCommand constructors (validate, csv, clusterpolicy) — 100%: command name/usage, subcommand presence (not an exact count), and the input flag located by name with its type (*cli.StringFlag), names, usage, and default "-" asserted.

Deliberately not covered (and why)

• Success paths of validateImage/validateImages — they construct a regclient internally and call ManifestGet against a real container registry, so they aren't hermetically unit-testable without a production refactor (injectable manifest getter). Tracked as follow-up.

The subtree's package totals are modest by design (validate 100%, csv 55.4%, clusterpolicy 26.9%); the uncovered remainder is the registry/entry-point code above.

Follow-ups (production changes, out of scope for this tests-only PR)

• Guard csv.validateImages against a CSV with no DeploymentSpecs/Containers (currently indexes [0] unchecked → possible panic on a malformed CSV).
• Decide validateALMExample(nil) handling (currently dereferences csv.Annotations).
• Inject the registry/manifest getter so success, registry-failure and context-cancellation paths become hermetically testable; consolidate the duplicated validateImage; wrap nested errors with %w.

Checklist

• No secrets, sensitive information, or unrelated changes
• Lint checks passing (make lint)
• Generated assets in-sync (make validate-generated-assets)
• Go mod artifacts in-sync (make validate-modules)
• Test cases are added for new code paths

Testing

go test ./cmd/gpuop-cfg/validate/... -covermode=count
golangci-lint run ./cmd/gpuop-cfg/validate/...           # 0 issues (GOOS=linux)

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[tariq1890]

Can you rebase this PR and squash the commit history?

[abrarshivani]

@tariq1890 I have rebased and squashed the commits. Can you please take look?