Re-enable event_tests and validation_tests modules disabled due to compile errors

[RUKAYAT-CODER]

Background

lib.rs currently comments out two test modules rather than fixing them:

// TODO: Fix event_tests module compilation errors (pre-existing issue)
// mod event_tests;
...
// TODO: Fix validation_tests compilation errors (pre-existing issue)
// mod validation_tests;

Both event_tests.rs and validation_tests.rs still exist on disk with real test cases in them, but since the mod declarations are commented out, cargo test never compiles or runs them. This means event-system and validation-module regressions can land without any of these existing tests catching them — the coverage is silently zero, not just reduced.

Implementation Plan

• Uncomment mod event_tests; and mod validation_tests; in lib.rs and run cargo test to surface the actual compile errors.
• Fix the compile errors in event_tests.rs (likely stale imports/signatures vs. current events.rs API).
• Fix the compile errors in validation_tests.rs (likely stale imports/signatures vs. current validation.rs API).
• Confirm both modules build and their tests pass under cargo test -p teachlink-contract.
• Add a CI check (or note in CONTRIBUTING.md) that no test module may be commented out to "fix" a red build — broken tests must be fixed or deleted, not silenced.

Acceptance Criteria

• event_tests and validation_tests are active mod declarations in lib.rs
• cargo test compiles and runs both modules with no ignored/skipped tests introduced as a workaround
• All tests in both modules pass