Fix broken integration tests in credential_flow.rs — wrong arg count, missing course setup

[DeFiVC]

What

Multiple integration tests in tests/integration/credential_flow.rs have compilation errors and logical bugs that would cause unexpected panics.

Why

1. credential_client.get_credentials_for(learner) is called with 1 argument but the function requires 3: (learner, start, limit).
2. Several tests (test_public_verification, test_multiple_course_credentials, test_credential_metadata_uri, test_credential_has_issuance_timestamp) mint credentials without ever creating courses first. The credential-nft contract calls tracker.course_exists(course_id) which returns false, causing unexpected panics.

Scope

• Fix get_credentials_for call to include start and limit arguments
• Add proper course creation and completion steps before each mint_credential call
• Ensure all tests compile and pass

Technical Context

• File: tests/integration/credential_flow.rs
• The credential-nft mint function verifies course existence via the progress-tracker contract
• Tests need a full flow: create course → enroll → complete modules/quizzes → mint credential

Acceptance Criteria

• All test functions compile
• get_credentials_for calls include correct argument count
• Each test sets up necessary course data before minting credentials
• Tests pass with cargo test

[addnad]

gone through the description and have a solid understanding of what's needed. can a pr in 24hours