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

[rubenhoenle]

Description

relates to STACKITCLI-273 / #893

Testing

1. With no application load balancers available in your STACKIT project verify the output of the application load balancer list command:
   • stackit beta alb list -> Expected output: No load balancers found for project "dev-tools-e2e-test"
   • stackit beta alb list --output-format json -> expected valid JSON output
   • stackit beta alb list --output-format YAML -> expected valid YAML output
2. With no application load balancer observability credential present, verify the output of the credentials list command:
   • stackit beta alb observability-credentials list -> Expected output: no credentials found
   • stackit beta alb observability-credentials list --output-format json -> expected valid JSON output
   • stackit beta alb observability-credentials list --output-format yaml -> expected valid YAML output
3. Create some observability credential: stackit beta alb observability-credentials add --displayname "abc" --username "abc" --password "abc"
4. With some application load balancer observability credential present, verify the output of the credentials list command again:
   • stackit beta alb observability-credentials list -> Expected output: Table showing the available credentials
   • stackit beta alb observability-credentials list --output-format json -> expected valid JSON output
   • stackit beta alb observability-credentials list --output-format yaml -> expected valid YAML output
5. Verify the output of the plans list command:
   • stackit beta alb plans -> Expected output: Table showing the available plans
   • stackit beta alb plans --output-format json -> expected valid JSON output
   • stackit beta alb plans --output-format YAML -> expected valid YAML output
6. Cleanup:
   • Delete the observability credential: stackit beta alb observability-credentials delete credentials-xxxx

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]

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[github-actions]

Merging this branch changes the coverage (2 decrease, 1 increase)

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/beta/alb/list	34.62% (-5.77%)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/beta/alb/observability-credentials/list	53.66% (+2.44%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/beta/alb/plans	34.15% (-7.32%)	👎

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/beta/alb/list/list.go	34.62% (-5.77%)	52	18 (-3)	34 (+3)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/beta/alb/observability-credentials/list/list.go	53.66% (+2.44%)	41	22 (+1)	19 (-1)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/beta/alb/plans/plans.go	34.15% (-7.32%)	41	14 (-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/beta/alb/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/beta/alb/plans/plans_test.go