enable strict JSON spec unmarshalling

[AndrewChubatiuk]

related issue VictoriaMetrics/helm-charts#2882

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Summary by cubic

Enable strict JSON unmarshalling for all Operator CR specs to reject unknown fields and return clear errors. This prevents typos or unsupported keys from being silently accepted.

• Bug Fixes

  • Added UnmarshalSpecStrict (uses DisallowUnknownFields) and wired all CR UnmarshalJSON paths (v1, v1alpha1, v1beta1) through it, setting ParsingSpecError on failure.
  • Updated VMAlertmanagerConfig tests/fixtures to use valid fields (e.g., msteams_configs, custom_fields placement).
  • Added a changelog entry documenting the behavior change.

• Migration

  • Manifests with unknown or misspelled spec fields will now be rejected by the webhook; fix or remove extra keys before applying.

^{Written for commit a984354. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 25 files

_{Re-trigger cubic}