fix(rabbitmq): print valid JSON/YAML output for list cmds

[rubenhoenle]

Description

relates to STACKITCLI-271 / #893

Testing

1. Verify the output of the plans list command:
   • stackit rabbitmq plans -> Expected output: Table showing the available RabbitMQ plans
   • stackit rabbitmq plans --output-format json -> expected valid JSON output
   • stackit rabbitmq plans --output-format yaml -> expected valid YAML output
2. With no RabbitMQ instance present in your project: verify the output of the instance list command:
   • stackit rabbitmq instance list -> Expected output: No instances found for project "xxx"
   • stackit rabbitmq instance list --output-format json -> expected valid JSON output
   • stackit rabbitmq instance list --output-format yaml -> expected valid YAML output
3. Create a RabbitMQ instance: stackit rabbitmq instance create --name my-instance --plan-name stackit-rabbitmq-1.2.10-replica --version 3.13, write the instance id to an env variable export INSTANCE_ID="xxx"
4. With a RabbitMQ instance present in your project: verify the output of the instance list command again:
   • stackit rabbitmq instance list -> Expected output: Table showing the available RabbitMQ instances in your project
   • stackit rabbitmq instance list --output-format json -> expected valid JSON output
   • stackit rabbitmq instance list --output-format yaml -> expected valid YAML output
5. Verify the output of the credentials list command with no credentials present:
   • stackit rabbitmq credentials list --instance-id $INSTANCE_ID -> Expected output: No credentials found for instance "my-instance"
   • stackit rabbitmq credentials list --instance-id $INSTANCE_ID --output-format json -> expected valid JSON output
   • stackit rabbitmq credentials list --instance-id $INSTANCE_ID --output-format yaml -> expected valid YAML output
6. Create a credential: stackit rabbitmq credentials create --instance-id $INSTANCE_ID
7. Verify the output of the credentials list command again with some credential(s) present:
   • stackit rabbitmq credentials list --instance-id $INSTANCE_ID -> Expected output: Table showing the available credentials for that RabbitMQ instance
   • stackit rabbitmq credentials list --instance-id $INSTANCE_ID --output-format json -> expected valid JSON output
   • stackit rabbitmq credentials list --instance-id $INSTANCE_ID --output-format yaml -> expected valid YAML output
8. Cleanup: Delete the RabbitMQ instance: stackit rabbitmq instance delete $INSTANCE_ID

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/create	56.90% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/delete	36.59% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/describe	56.82% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/list	60.00% (+6.00%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/create	60.20% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/delete	25.58% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/describe	58.33% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/list	57.69% (+5.77%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/update	57.73% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/plans	53.85% (+5.77%)	👍

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/list/list.go	60.00% (+6.00%)	50	30 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/list/list.go	57.69% (+5.77%)	52	30 (+3)	22 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/plans/plans.go	53.85% (+5.77%)	52	28 (+3)	24 (-3)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/describe/describe_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/credentials/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/describe/describe_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/instance/update/update_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/rabbitmq/plans/plans_test.go