Document nullable response dates and pending add-ons

[javorosas]

Summary

• Document nullable response dates for customers, invoices, retentions, and organization certificates.
• Document plan, add_ons, pending_plan_update, and pending_add_ons_update returned by the organization API.
• Align the OpenAPI v2 response schema with Facturapi Node SDK 6.0 changes in FAC-2305 Hydrate response dates consistently in SDK 6.0 facturapi-node#120.

Validation

• Parsed website/openapi_v2.yaml and asserted the changed schema fields.
• git diff --check.

[javorosas]

Superseded by #303, which consolidates the response-schema corrections with the broader API documentation alignment in both languages.