fix(project): validate billingReference for standalone public projects

[eloiberlinger1]

Description

relates to #1452

Before the fix:

 ~/Documents/dev/stackit-cli  main ?1                                                                                                                                          3s

> ./stackit-test project create --parent-id 12345 --name "Test public project" --label scope=PUBLIC

Are you sure you want to create a project under the parent with ID "12345"? [y/N] y
Error: create project: 404 Not Found, status code 404, Body: {"timeStamp":"2026-07-05T19:52:13.783789128Z","path":"/resource-management/v2/projects","status":404,"error":"Not Found","message":"Container [12345] was not found."}

After the fix:

 ~/Documents/dev/stackit-cli  fix-project-public-validation ?1                                                                                                                

> ./stackit-test project create --parent-id 12345 --name "Test public project" --label scope=PUBLIC

Error: the provided flag --label is invalid: creating a standalone public project (without a network area) requires a 'billingReference' label (e.g., --label billingReference=<value>)

> 

Note on Testing

Unfortunately because I only have a French phone number, I was unable to complete the project creation process to access the STACKIT Portal to check the payload validation conditions.

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)

[Copilot]

Pull request overview

This PR adds client-side validation to stackit project create so that creating a standalone PUBLIC project (i.e., without a Network Area) fails early with a clear error when the required billingReference label is missing, aligning CLI behavior with the governance requirements described in #1452.

Changes:

• Parse --network-area-id early during input parsing to support conditional label validation.
• Add validation: if scope=PUBLIC and no network area is provided, require a billingReference label.

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