Validates and initializes billing for a leasing contract without requiring a signature.

This endpoint is useful for:

• Creating or recreating billing items after contract setup
• Validating billing configuration for an existing contract
• Manually triggering the first rent call when needed
• Testing billing logic without going through the signature workflow

What this endpoint does:

1. Creates initial billing items for the leasing contract
2. Calls the first rent for the month of the lease start date
3. Generates due date notices if applicable
4. Does NOT send tenant notifications or update the signed_at timestamp

Note: This is a direct billing validation endpoint, separate from the
post-signature workflow. It's ideal for API-driven workflows that don't
involve electronic signatures, or for manual billing validation.

Billing Requirements:

• Contract must have a valid start_date
• Contract must have billing configuration (tenant_fees, deposit, etc.)
• All required relationships (asset, GLI provider, etc.) must be in place