fix(mongodbflex): print valid JSON for list cmds

[rubenhoenle]

Description

relates to STACKITCLI-244

Testing

1. Without any mongodb instance present in your project:
   • stackit mongodbflex instance list -> should print human-readable message that no instances were found
   • stackit mongodbflex instance list -o json -> should print valid JSON output
   • stackit mongodbflex instance list -o yaml -> should print valid YAML output
2. Create a mongodb instance: stackit mongodbflex instance create --name my-instance --cpu 1 --ram 4 --acl 0.0.0.0/0
3. Run the list command again
   • stackit mongodbflex instance list -> should print a table with all mongodb instances which were found
   • stackit mongodbflex instance list -o json -> should print valid JSON output
   • stackit mongodbflex instance list -o yaml -> should print valid YAML output
4. Set env var for instance id: export INSTANCE_ID="6c709d4e-3d1c-4931-9667-dac42ff9043f"
5. List users for the instance:
   • stackit mongodbflex user list --instance-id $INSTANCE_ID -> should print human-readable message that no users were found
   • stackit mongodbflex user list --instance-id $INSTANCE_ID -o json -> should print valid JSON output
   • stackit mongodbflex user list --instance-id $INSTANCE_ID -o yaml -> should print valid YAML output
6. Create a user: stackit mongodbflex user create --instance-id $INSTANCE_ID --role read --database default
7. Run the list users command again:
   • stackit mongodbflex user list --instance-id $INSTANCE_ID -> should print a table of users which were found
   • stackit mongodbflex user list --instance-id $INSTANCE_ID -o json -> should print valid JSON output
   • stackit mongodbflex user list --instance-id $INSTANCE_ID -o yaml -> should print valid YAML output
8. List backups for the instance:
   • stackit mongodbflex backup list --instance-id $INSTANCE_ID -> should print human-readable message that no backups were found
   • stackit mongodbflex backup list --instance-id $INSTANCE_ID -o json -> should print valid JSON output
   • stackit mongodbflex backup list --instance-id $INSTANCE_ID -o yaml -> should print valid YAML output
     9. Create a backup: (Not possible, there's no reliable way to create a backup on demand)
     10. Run the list backups command again
     - stackit mongodbflex backup list --instance-id $INSTANCE_ID -> should print a table of backups which were found
     - stackit mongodbflex backup list --instance-id $INSTANCE_ID -o json -> should print valid JSON output
     - stackit mongodbflex backup list --instance-id $INSTANCE_ID -o yaml -> should print valid YAML output
9. Cleanup test resources:
   • Delete the user: stackit mongodbflex user delete 20fee51d-763c-45ea-8196-ac1147db3b90 --instance-id $INSTANCE_ID
   • Delete the instance: stackit mongodbflex instance delete $INSTANCE_ID

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/mongodbflex/backup/list	42.11% (-5.26%)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/mongodbflex/instance/list	57.45% (+6.38%)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/mongodbflex/user/list	60.00% (+6.00%)	👍

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/mongodbflex/backup/list/list.go	42.11% (-5.26%)	57	24 (-3)	33 (+3)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/mongodbflex/instance/list/list.go	57.45% (+6.38%)	47	27 (+3)	20 (-3)	👍
github.com/stackitcloud/stackit-cli/internal/cmd/mongodbflex/user/list/list.go	60.00% (+6.00%)	50	30 (+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/mongodbflex/backup/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/mongodbflex/instance/list/list_test.go
• github.com/stackitcloud/stackit-cli/internal/cmd/mongodbflex/user/list/list_test.go