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

[rubenhoenle]

Description

relates to STACKITCLI-267 / #893

Testing

1. With no object storage bucket present in your project, verify the bucket list command output:
   • stackit object-storage bucket list -> Expected output: No buckets found for project xxx
   • stackit object-storage bucket list --output-format json -> expected: valid JSON output
   • stackit object-storage bucket list --output-format yaml -> expected: valid YAML output
2. Create an object storage bucket: stackit object-storage bucket create my-bucket
3. With an object storage bucket present in your project, verify the bucket list command output again:
   • stackit object-storage bucket list -> Expected output: Table showing the available object storage buckets
   • stackit object-storage bucket list --output-format json -> expected: valid JSON output
   • stackit object-storage bucket list --output-format yaml -> expected: valid YAML output
4. Verify the output of the credentials group list command (FYI: it's not possible to verify this for the case where no object storage credentials group is present in the project because there's always a default group present)
   • stackit object-storage credentials-group list -> Expected output: Table showing the available object storage credentials groups
   • stackit object-storage credentials-group list --output-format json -> expected: valid JSON output
   • stackit object-storage credentials-group list --output-format yaml -> expected: valid YAML output
5. Choose one from the existing credentials groups (with existing credentials in them) and verify the credentials list command for them:
   • stackit object-storage credentials list --credentials-group-id xxx -> Expected output: Table showing the available credentials
   • stackit object-storage credentials list --credentials-group-id xxx --output-format json -> expected: valid JSON output
   • stackit object-storage credentials list --credentials-group-id xxx --output-format yaml -> expected: valid YAML output
6. Create a new credential group (stackit object-storage credentials-group create --name example), then verify the credential list command output for it again
   • stackit object-storage credentials list --credentials-group-id xxx -> Expected output: No credentials found for credentials group "example"
   • stackit object-storage credentials list --credentials-group-id xxx --output-format json -> expected: valid JSON output
   • stackit object-storage credentials list --credentials-group-id xxx --output-format yaml -> expected: valid YAML output
7. Cleanup:
   • Delete the credentials group you created: stackit object-storage credentials-group delete xxx
   • Delete the object storage bucket: stackit object-storage bucket delete my-bucket

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 changes the coverage (1 decrease, 2 increase)

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/bucket/create	37.50% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/bucket/delete	28.21% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/bucket/describe	66.67% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/bucket/list	42.86% (-6.12%)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials-group/create	55.26% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials-group/delete	33.33% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials-group/list	62.79% (+6.98%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials/create	57.69% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials/delete	38.10% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials/list	61.54% (+5.77%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/disable	28.57% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/enable	28.57% (ø)

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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/object-storage/bucket/list/list.go	42.86% (-6.12%)	49	21 (-3)	28 (+3)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials-group/list/list.go	62.79% (+6.98%)	43	27 (+3)	16 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials/list/list.go	61.54% (+5.77%)	52	32 (+3)	20 (-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/object-storage/bucket/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/bucket/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/bucket/describe/describe_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/bucket/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials-group/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials-group/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials-group/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/credentials/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/disable/disable_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/object-storage/enable/enable_test.go