test(integration): add deposit → withdraw unused collateral integration flow

[BarryArinze]

Summary

Closes #557

This PR expands the integration test harness by covering the complete happy-path workflow for depositing collateral and subsequently withdrawing unused collateral. The new test validates that the end-to-end flow behaves correctly under normal operating conditions and protects against regressions in state transitions.

What Changed

• Added (or extended) the integration test in tests/market_test.rs.
• Implemented a complete deposit → withdraw_unused_collateral workflow.
• Verified state transitions complete successfully throughout the lifecycle.
• Added optional assertions for fee handling where applicable.
• Integrated the test with the existing CI suite.

Why

While individual contract functions are already tested, integration tests provide confidence that multiple operations work together correctly. This scenario represents a common user workflow and should remain protected by regression tests.

The new test ensures:

• Deposits are processed successfully.
• Unused collateral can be withdrawn afterward.
• Contract state remains consistent across operations.
• Future changes cannot unintentionally break the end-to-end flow.

Testing

• ✅ Added integration coverage for deposit followed by withdraw_unused_collateral.
• ✅ Verified the happy path executes successfully.
• ✅ Added optional fee assertions where supported.
• ✅ Confirmed compatibility with the existing CI pipeline.

Acceptance Criteria

• Happy-path integration test added.
• Deposit and withdrawal execute successfully.
• Fee assertions included where applicable.
• CI passes successfully.
• No unrelated contract behavior modified.