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

[rubenhoenle]

Description

relates to STACKITCLI-264 / #893

Testing

1. With no logme instance present in your project: Verify output of list cmd
   • stackit logme instance list -> Expected output: No instances found for project "xxx"
   • stackit logme instance list --output-format json -> Should output valid JSON
   • stackit logme instance list --output-format yaml -> Should output valid YAML
2. Create a logme instance in your project: stackit logme instance create --name my-instance2 --plan-name stackit-logme2-1.2.50-replica --version 2
3. With a logme instance present in your project: Verify output of list cmd again
   • stackit logme instance list -> Expected output: Table showing the logme instance(s)
   • stackit logme instance list --output-format json -> Should output valid JSON
   • stackit logme 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 being present for your logme instance: Verify output of list cmd
   • stackit logme credentials list --instance-id $INSTANCE_ID -> Expected output: `No credentials found for instance "my-instance2"
   • stackit logme credentials list --instance-id $INSTANCE_ID --output-format json -> Sould output valid JSON
   • stackit logme credentials list --instance-id $INSTANCE_ID --output-format yaml -> Sould output valid YAML
6. Create a credential for your logme instance: stackit logme credentials create --instance-id $INSTANCE_ID
7. With a credential being present for your logme instance: Verify output of list cmd again
   • stackit logme credentials list --instance-id $INSTANCE_ID -> Expected output: Table showing the existing credentials
   • stackit logme credentials list --instance-id $INSTANCE_ID --output-format json -> Sould output valid JSON
   • stackit logme credentials list --instance-id $INSTANCE_ID --output-format yaml -> Sould output valid YAML
8. Verify plan list output:
   • stackit logme plans -> Expected output: A table showing the available plans
   • stackit logme plans --output-format json -> Should output valid JSON
   • stackit logme plans --output-format yaml -> Should output valid YAML
9. Cleanup: Delete the mariadb instance: stackit logme 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/logme/credentials/list	60.00% (+6.00%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/logme/instance/list	56.86% (+5.88%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/logme/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/logme/credentials/list/list.go	60.00% (+6.00%)	50	30 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/logme/instance/list/list.go	56.86% (+5.88%)	51	29 (+3)	22 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/logme/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/logme/credentials/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/logme/instance/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/logme/plans/plans_test.go