Enable SAPIPY to use less than 4 database nodes

[ckunki]

Problem

The generated OpenAPI client already supports the optional numNodes field for database creation, but SAPIPY's handwritten helper layer did not expose it. As a result, users of OpenApiAccess could not request databases with fewer than 4 nodes through the convenience API.

Expected behavior

The handwritten helpers should allow callers to pass a database node count through to the SaaS API.

Scope

• add optional num_nodes support to OpenApiAccess.create_database()
• forward num_nodes through OpenApiAccess.database()
• cover the forwarding path with unit tests
• add an integration test for creating a 2-node database and verifying the reported settings

Implementation

Implemented in #167.

Validation completed:

• poetry run pytest test/unit/test_api_access.py in Lima VM default
• live integration execution still requires SaaS credentials and provisions a real database