Include missing default configuration name in error

[ajavanma]

Description

When config-manager selects an explicit default that does not exist, its error currently omits the profile name (specified config does not exist). Report the requested default name and add regression coverage for missing/existing defaults and explicit node selections, preserving selection precedence and failure behavior.

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

• New TestUpdateConfigName fails on unchanged upstream only for the missing-default diagnostic; its four control cases pass.
• go test ./cmd/config-manager -count=1 -v passes after the fix.
• make fmt, make check (0 lint issues), and make test pass with Go 1.27.1 on Linux ARM64. make test also builds all packages and four commands.
• make check-modules and make check-third-party-notices pass; notices were regenerated for Linux AMD64 and ARM64.

No Kubernetes/GPU end-to-end tests were run. Module and third-party notice regeneration checks also pass on this commit, with no file differences. The module check passed after retrying transient dependency-download EOF failures. The build emits deprecation warnings from vendored NVML C bindings.

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[myeolenv]

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