Import tenants from CSV file

Import tenants from a CSV file using upsert semantics.

Expected CSV format:

• tenant_key (required): Unique tenant identifier
• tenant_name (optional): Human-readable name
• group_names (optional): Comma-separated list of group names
• Additional columns can be mapped to attributes via attribute_columns

Creates new tenants and updates existing ones with new name/groups. Groups are added additively (not replaced) and auto-created if needed.

Optionally processes attribute columns: values are validated against attributes.yaml, stored additively, and validation errors are non-fatal.

RLS: Filtered to current client (ClientRLSDB).

Raises: ValidationError: If CSV format is invalid (400) TenantCreationNotAllowedError: If tenancy type is NONE (400)