fix: improve error message when adoption-fields annotation is missing

[shinichitazawa]

Description

When using services.k8s.aws/adoption-policy: adopt without the services.k8s.aws/adoption-fields annotation, ExtractAdoptionFields in pkg/runtime/util.go calls json.Unmarshal on an empty string, producing the generic Go error unexpected end of JSON input.

This error gives no indication that the adoption-fields annotation is missing, making it very difficult to diagnose — especially since it becomes a TerminalError that permanently blocks reconciliation.

Changes

• Added an explicit empty-string check in ExtractAdoptionFields that returns a descriptive error message explaining that the adoption-fields annotation is required when adoption-policy is set to adopt
• Wrapped json.Unmarshal error with fmt.Errorf and %w to provide context about which annotation failed to parse
• Added two unit tests for the new error cases

Before

{"level":"error","msg":"Reconciler error","error":"unexpected end of JSON input"}

After

{"level":"error","msg":"Reconciler error","error":"services.k8s.aws/adoption-fields annotation is required when adoption-policy is set to 'adopt'; please provide the resource identifier fields as a JSON object (e.g. '{\"name\": \"my-resource\"}')"}

Testing

• Unit tests added and passing (go test ./pkg/runtime/...)
• Manually verified with ACK IAM Controller v1.6.2 on EKS

Fixes aws-controllers-k8s/community#2824

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ack-prow]

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[ack-prow]

Hi @shinichitazawa. Thanks for your PR.

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