Add authorization tests for admin functions

[SrvFernandes]

Closes #495

Summary

Adds negative authorization tests for all admin functions in progress-tracker and credential-nft contracts that were previously untested.

Changes

• Add tests/integration/progress_tracker_auth_tests.rs with non-admin auth tests for:
  • create_course
  • archive_course
  • set_course_content_hash
  • set_course_difficulty
  • set_course_tags
  • update_course_version
  • set_prerequisites
  • emergency_pause
• Add tests/integration/credential_auth_tests.rs with non-admin auth tests for:
  • mint_credential
  • renew_credential
  • set_credential_display
  • transfer_admin
• Each test verifies authorization failure without using mock_all_auths()

Testing

All new tests pass in CI, verifying that non-admin callers are properly rejected with authorization errors.

Bounty payout address (Base / EVM): 0x96eE7904BdCd8a82c71B4FFc3362C96b1Aae03e0
Bounty payout address (Stellar / Soroban): GCTRCN2H6EVVRQH4MKHVWMTY2SPC4ZTRHQZQOSKF5PXFRA4TNDGGF4VL