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

[rubenhoenle]

Description

relates to STACKITCLI-269 / #893

Testing

1. Validate output of plans list command:
   • stackit opensearch plans -> Expected output: Table showing the available opensearch plans
   • stackit opensearch plans --output-format json -> expected valid JSON output
   • stackit opensearch plans --output-format yaml -> expected valid YAML output
2. Validate the output of the instance list command with no opensearch instance present in your project:
   • stackit opensearch instance list -> Expected output: No instances found for project "xxx"
   • stackit opensearch instance list --output-format json -> expected valid JSON output
   • stackit opensearch instance list --output-format yaml -> expected valid YAML output
3. Create an opensearch instance: stackit opensearch instance create --name my-instance --plan-name stackit-opensearch-1.2.10-replica --version 2
4. Save the instance id of the opensearch instance you just created into an env variable: export INSTANCE_ID="xxx"
5. Validate the output of the instance list command with the opensearch instance present in your project:
   • stackit opensearch instance list -> Expected output: Table showing the available opensearch instances
   • stackit opensearch instance list --output-format json -> expected valid JSON output
   • stackit opensearch instance list --output-format yaml -> expected valid YAML output
6. With no credentials available for your opensearch instance: Validate the output of the credentials list command
   • stackit opensearch credentials list --instance-id $INSTANCE_ID -> Expected output: No credentials found for instance "my-instance"
   • stackit opensearch credentials list --instance-id $INSTANCE_ID -> expected valid JSON output
   • stackit opensearch credentials list --instance-id $INSTANCE_ID -> expected valid YAML output
7. Create a credential for your opensearch instance: stackit opensearch credentials create --instance-id $INSTANCE_ID
8. With some credential(s) available for your opensearch instance: Validate the output of the credentials list command again
   • stackit opensearch credentials list --instance-id $INSTANCE_ID -> Expected output: Table showing the available credentials
   • stackit opensearch credentials list --instance-id $INSTANCE_ID -> expected valid JSON output
   • stackit opensearch credentials list --instance-id $INSTANCE_ID -> expected valid YAML output
9. Delete the opensearch instance: stackit opensearch 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]

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[github-actions]

Merging this branch changes the coverage (1 decrease, 2 increase)

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/create	55.77% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/delete	36.59% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/describe	56.82% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/list	60.00% (+6.00%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/create	59.78% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/delete	25.58% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/describe	52.08% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/list	40.43% (-6.38%)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/update	57.73% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/plans	55.77% (+5.77%)	👍

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/list/list.go	60.00% (+6.00%)	50	30 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/list/list.go	40.43% (-6.38%)	47	19 (-3)	28 (+3)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/plans/plans.go	55.77% (+5.77%)	52	29 (+3)	23 (-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/opensearch/credentials/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/describe/describe_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/credentials/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/describe/describe_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/instance/update/update_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/opensearch/plans/plans_test.go