fix(mariadb): print valid JSON/YAML for list commands

[rubenhoenle]

Description

relates to STACKITCLI-263 / #893

Testing instructions

1. With no mariadb instance present in your project: Verify output of list cmd
   • stackit mariadb instance list -> Expected output: No instances found for project "xxx"
   • stackit mariadb instance list --output-format json -> Should output valid JSON
   • stackit mariadb instance list --output-format yaml -> Should output valid YAML
2. Create a mariadb instance in your project: stackit mariadb instance create --name my-instance --plan-name stackit-mariadb-1.2.10-replica --version 10.6
3. With a mariadb instance present in your project: Verify output of list cmd again
   • stackit mariadb instance list -> Expected output: Table showing the mariadb instance(s)
   • stackit mariadb instance list --output-format json -> Should output valid JSON
   • stackit mariadb instance list --output-format yaml -> Should output valid YAML
4. Store the instance id into a env variable: export INSTANCE_ID="xxx"
5. With no credentials present for your mariadb instance: verify output of list cmd
   • stackit mariadb credentials list --instance-id $INSTANCE_ID -> Expected output: No credentials found for instance "my-instance"
   • stackit mariadb credentials list --instance-id $INSTANCE_ID --output-format json -> Should output valid JSON
   • stackit mariadb credentials list --instance-id $INSTANCE_ID --output-format yaml -> Should output valid YAML
6. Create a credential: stackit mariadb credentials create --instance-id $INSTANCE_ID
7. With a credential present for your mariadb instance: verify output of list cmd again
   • stackit mariadb credentials list --instance-id $INSTANCE_ID -> Expected output: Table showing the credential(s)
   • stackit mariadb credentials list --instance-id $INSTANCE_ID --output-format json -> Should output valid JSON
   • stackit mariadb credentials list --instance-id $INSTANCE_ID --output-format yaml -> Should output valid YAML
8. Verify plan list output:
   • stackit mariadb plans -> Expected output: A table showing the available plans
   • stackit mariadb plans --output-format json -> Should output valid JSON
   • stackit mariadb plans --output-format yaml -> Should output valid YAML
9. Cleanup: Delete the mariadb instance: stackit mariadb 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/mariadb/credentials/list	60.00% (+6.00%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/mariadb/instance/list	56.86% (+5.88%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/mariadb/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/mariadb/credentials/list/list.go	60.00% (+6.00%)	50	30 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/mariadb/instance/list/list.go	56.86% (+5.88%)	51	29 (+3)	22 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/mariadb/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/mariadb/credentials/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/mariadb/instance/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/mariadb/plans/plans_test.go