test(integration): add concurrent credential and reward claim integration test

[lmduy2612]

Description

Fixes #640

This PR adds cross-contract integration test coverage in tests/integration/full_flow.rs verifying that token reward claiming and credential NFT minting operate reliably in the same transaction session without interference:

1. test_claim_reward_then_mint_credential_sequence:

   • Executes reward claims (quiz_midterm + quiz_final) first, bringing token balance and total_supply to 16,000.
   • Subsequently mints credential NFT for the learner.
   • Verifies credential validity, non-revocation status, and strictly consistent token balance and total supply.

2. test_mint_credential_then_claim_reward_reverse_sequence:

   • Executes mint_credential first while token balance and supply are 0.
   • Subsequently claims token rewards for the completed quizzes.
   • Verifies that token rewards are properly credited (16,000) and that the previously minted credential remains intact.

3. test_concurrent_multi_learner_claim_and_mint_same_session:

   • Executes interleaved reward claiming and credential minting across multiple learners (learner1 and learner2) in the same environment session without state reset.
   • Verifies isolation between learners, correct cumulative supply (32,000), and unique credential issuance without race conditions.

Verification

• Run full suite: cargo test --test full_flow (All 4 tests passed)
• Code formatted with rustfmt.