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

[rubenhoenle]

Description

relates to STACKITCLI-266 / #893

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 decrease overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/create	56.34% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/delete	28.30% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/describe	70.59% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/list	69.61% (-2.94%)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/update	47.67% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/clone	46.43% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/create	57.14% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/delete	24.44% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/describe	77.05% (ø)
github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/list	70.97% (-3.23%)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/update	50.72% (ø)

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/list/list.go	69.61% (-2.94%)	102	71 (-3)	31 (+3)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/list/list.go	70.97% (-3.23%)	93	66 (-3)	27 (+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/dns/record-set/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/describe/describe_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/record-set/update/update_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/clone/clone_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/create/create_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/delete/delete_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/describe/describe_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/dns/zone/update/update_test.go