[sap-document-ai]: Multitenancy Support

[samyuktaprabhu]

The plugin currently supports single-tenancy only. The goal of this issue is to add multitenancy support. In multi-tenant applications, each subscriber tenant has its own Document AI service instance and quota.

The plugin should:

• Support tenant-aware credential resolution (e.g. via the SAP Service Manager).
• Ensure extraction jobs and stored documents are isolated per tenant.
• Handle custom schema synchronisation per tenant for custom document types.

[github-actions]

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