Pull Request: Comprehensive QA Suite & Interaction Documentation (#339-350)

[JerryIdoko]

📝 Description
This PR significantly hardens the teachLink smart contract by introducing a rigorous multi-layered testing strategy. We have moved from basic functional tests to a comprehensive suite that covers edge-case errors, state machine integrity, and high-concurrency safety. Additionally, we've added a developer-facing guide for interacting with the contract.

🎯 Key Changes by Issue

1. Error Scenario Testing (Add comprehensive error scenario tests #339)
   Path Coverage: Added tests for every panic! and Error variant in the contract.

Message Validation: Verified that error strings/codes returned to the caller match the expected failure reasons.

2. State Transition & Invariants (Add state transition tests #340)
   Valid Flows: Tested the full lifecycle of a link (e.g., Draft -> Active -> Completed).

Invariant Checks: Implemented checks to ensure that balances and ownership data remain consistent even if a transition is rejected.

3. Concurrency & Race Conditions (Add comprehensive concurrency tests #344)
   Simulated Contention: Created tests that invoke the same contract function from multiple mock accounts in the same ledger footprint to verify Soroban's transaction isolation.

Locking Logic: Verified that parallel attempts to update a single "Link" resource fail gracefully or sequence correctly.

4. Interaction Examples & Best Practices (Add comprehensive contract interaction examples #350)
   Guide: Created examples/contract_interactions.md detailing how to use the CLI and SDK to perform common operations.

Integration Patterns: Documented how to handle the error scenarios tested in #339 from a frontend/off-chain perspective.

✅ Acceptance Criteria Checklist
[x] Error Paths: 100% of defined Error enums are triggered in tests.

[x] State Integrity: Invalid transitions (e.g., jumping from Draft to Completed) are successfully rejected.

[x] Thread Safety: Concurrency tests pass without state corruption or unexpected panics.

[x] Documentation: Examples include both successful and failing transaction JSONs.

🚀 How to Verify
Run Test Suite:

Bash
cargo test --test error_scenarios_test
cargo test --test state_transitions_test
cargo test --test concurrency_test
Review Examples: Open examples/contract_interactions.md to ensure the integration patterns align with the current ABI.

🔗 Linked Issues
Closes #339,
Closes #340,
Closes #344,
Closes #350