fix(load-balancer): print valid JSON/YAML output for list cmds

[j1n-o9r]

Description

relates to STACKITCLI-241 / #893

Testing Instructions

• stackit load-balancer list -> Expected: No load balancers found for project "xxx"

• stackit load-balancer list --output-format json -> Should output valid JSON

• stackit load-balancer list --output-format yaml -> Should output valid YAML

• stackit load-balancer observability-credentials list -> Expected: No observability credentials found for Load Balancer on project "xxx"

• stackit load-balancer observability-credentials list --output-format json -> Should output valid JSON

• stackit load-balancer observability-credentials list --output-format yaml -> Should output valid YAML

Test with --used flag

• stackit load-balancer observability-credentials list --used -> Expected: No used observability credentials found for Load Balancer on project "xxx"
• stackit load-balancer observability-credentials list --output-format json --used -> Should output valid JSON
• stackit load-balancer observability-credentials list --output-format yaml --used -> Should output valid YAML

Test with --unused flag

• stackit load-balancer observability-credentials list --unused -> Expected: No unused observability credentials found for Load Balancer on project "xxx"
• stackit load-balancer observability-credentials list --output-format json --unused -> Should output valid JSON
• stackit load-balancer observability-credentials list --output-format yaml --unused -> Should output valid YAML

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/load-balancer/list	58.49% (+5.66%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/load-balancer/observability-credentials/list	52.86% (+4.97%)	👍

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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/load-balancer/list/list.go	58.49% (+5.66%)	53	31 (+3)	22 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/load-balancer/observability-credentials/list/list.go	52.86% (+4.97%)	70 (-1)	37 (+3)	33 (-4)	👍

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/load-balancer/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/load-balancer/observability-credentials/list/list_test.go