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

[j1n-o9r]

Description

relates to STACKITCLI-268 / #893

Testing Instructions

Observability Plans:

• stackit observability plans -> Expected: No plans found for project "xxx"
• stackit observability plans --output-format json -> Should output valid JSON
• stackit observability plans --output-format yaml -> Should output valid YAML

Observability Instance:

• stackit observability instance list -> Expected: No servers found for project "xxx"
• stackit observability instance list --output-format json -> Should output valid JSON
• stackit observability instance list --output-format yaml -> Should output valid YAML

Create Observability Instance with ID yyy and name zzz

Observability Credentials:

• stackit observability credentials list --instance-id yyy -> Expected: No credentials found for instance "zzz"
• stackit observability credentials list --instance-id yyy --output-format json -> Should output valid JSON
• stackit observability credentials list --instance-id yyy --output-format yaml -> Should output valid YAML

Observability Scrape Config:

• stackit observability scrape-config list --instance-id yyy -> Expected: No scrape configurations found for instance "zzz"
• stackit observability scrape-config list --instance-id yyy --output-format json -> Should output valid JSON
• stackit observability scrape-config list --instance-id yyy --output-format yaml -> 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/observability/credentials/list	58.33% (+6.25%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/observability/instance/list	57.45% (+6.38%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/observability/plans	59.18% (+6.12%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/observability/scrape-config/list	55.56% (+5.56%)	👍

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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/observability/credentials/list/list.go	58.33% (+6.25%)	48	28 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/observability/instance/list/list.go	57.45% (+6.38%)	47	27 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/observability/plans/plans.go	59.18% (+6.12%)	49	29 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/observability/scrape-config/list/list.go	55.56% (+5.56%)	54	30 (+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/observability/credentials/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/observability/instance/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/observability/plans/plans_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/observability/scrape-config/list/list_test.go