[sap-document-ai]: Add mock Document AI client for local development #102

Local development currently runs in a degraded mode: jobs are created in PENDING status, but no document extraction is performed because a Document AI service binding is required. This makes it difficult to develop and test the application locally without access to a real service instance.

Provide a mock implementation of the Document AI client that returns configurable, static extraction results without requiring a service binding.
This would enable local development, testing, and demos without relying on an actual Document AI service.

Possible approaches

• Implement a built-in mock client that returns static or configurable extraction results.
• Provide a mock CAP service that can be enabled via package.json profiles.
• Investigate integration with the CAP cds mock command.