Fix Helm effective namespace validation

[ajavanma]

Description

Validate the Helm chart's effective resource namespace so --namespace default --set namespaceOverride=gpu-system succeeds and the reverse override requires allowDefaultNamespace=true.

Use the existing namespace helper in one validation guard and remove the duplicate release-namespace guard. Preserve the error text and explicit opt-in. Add Go/Helm regression coverage for release namespaces with and without overrides, both with opt-in disabled and enabled, asserting errors and rendered DaemonSet namespaces.

Checklist

• No secrets, sensitive information, or unrelated changes
• Lint checks passing (make lint)
• Unit tests passing (make test)
• Go mod artifacts in-sync (make check-modules)
• Third-party notices in-sync (make check-third-party-notices)
• Test cases are added for new code paths

Testing

• Added regression test on the unchanged template: fails in the two override-direction cases, with 10 control cases passing.
• make test-helm: passes with the fix, including all 12 new regression cases.
• make fmt plus gofmt -w tests/helm/helm_template_test.go: passes, no unrelated changes.
• make check: passes, 0 lint issues.
• make test: passes, including package compilation and all four command builds. Vendored NVML code emits deprecation warnings.
• git diff --check: passes.
• Additional separate-module lint (cd tests && golangci-lint run --allow-serial-runners --timeout=5m ./helm/...) reports three modernize/newexpr suggestions in unchanged ptr code. Running the same lint against the base test file reproduces all three; no unrelated modernization is included.

Validated on Linux ARM64 with Go 1.27.1, Helm 4.3.0, and golangci-lint 2.13.2. GPU/cluster e2e tests were not run; this change is exercised through real Helm rendering without a cluster. Module and third-party-notice regeneration checks were not run because no dependencies or license inputs changed; their checklist boxes remain unchecked.

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[myeolenv]

Thanks for the contribution. We will review this PR soon.