Get Tableau projects using connection credentials

Fetch all Tableau projects from a Tableau server using provided connection details.

Two modes:

• Wizard (no datastore_id): caller supplies plaintext PAT in the body.
• Add-site modal (datastore_id present, PAT omitted): falls back to credentials stored on the existing data store. The caller-supplied site_name lets the customer browse projects on a new site without re-entering the PAT.

PAT credentials are treated atomically — if the caller supplies only one of pat_name/pat_secret, the stored pair replaces both. This prevents a partial override from silently pairing the caller's new name with the stored secret.

Returns a hierarchical list of projects.