[codex] Wrap generated OpenAPI models behind facade

[tkilias]

Summary

Replace generated OpenAPI model leakage from the handwritten client package with handwritten wrapper types.

What Changed

• added handwritten facade wrapper types for status, API errors, databases, clusters, connections, and allowed IPs
• updated the facade to convert generated OpenAPI responses into handwritten wrappers before returning them
• switched api_access.py and package-level PROMISING_STATES usage to the handwritten wrapper surface
• updated unit and integration-facing type imports to use the facade wrappers

Why

The previous facade still exposed generated classes from exasol.saas.client.openapi, which kept the generated API surface coupled to the handwritten client layer.

Impact

The handwritten client package now presents a stable wrapper surface instead of forwarding generated models directly.

Validation

• python -m py_compile exasol/saas/client/openapi_facade_types.py exasol/saas/client/openapi_facade.py exasol/saas/client/init.py exasol/saas/client/api_access.py test/unit/test_api_access.py test/unit/test_ensure_type.py test/integration/conftest.py test/integration/test_databases.py test/integration/test_allowed_ip.py
• pytest was not run in this environment because it is not installed

[ckunki]

Also a non-facade data type, I guess.
Will be solved by changing the return type of self.create_database().
Maybe we should add a type hint to this context manager?

[ckunki]

OK, thanks for answering my other comment.
Then only the type hint could be an optional improvement.