[OID4VCI]: Enforce batch_size ≥ 2 validation for batch_credential_issuance

[forkimenjeckayang]

Summary

This PR implements validation for the batch_credential_issuance.batch_size parameter to ensure compliance with the OID4VCI specification, which requires that batch_size MUST be 2 or greater.

Changes Made

Implementation

• Added validation in getBatchCredentialIssuance() method to check that batch_size is at least 2

Testing

• Added comprehensive test method testBatchCredentialIssuanceValidation() covering:
  • Valid cases: batch sizes 2, 5 (should be accepted)
  • Invalid cases: batch sizes -1, 0, 1 (should be rejected)
  • Edge case: batch size exactly 2 (minimum valid value)
  • Created reusable helper method testBatchSizeValidation() to reduce code duplication
  • Ensured tests don't interfere with each other by properly restoring original realm attributes

Addresses

• Issue Keycloak: Update batch_credential_issuance to enforce batch_size ≥ 2. #77

[IngridPuppet]

Some minor comments, please could you check?

[IngridPuppet]

LGTM