test(learn-token): add get_vesting_claimed unit tests (fixes #634)

[lmduy2612]

Closes #634.

Summary of Changes

Adds comprehensive unit tests for get_vesting_claimed in contracts/learn-token:

• Beneficiary with no vesting: verifies returning 0 (test_get_vesting_claimed_returns_zero_for_beneficiary_with_no_vesting).
• Before any claims: verifies returning 0 when a vesting schedule exists but before any claims are executed (test_get_vesting_claimed_returns_zero_before_any_claims).
• After claiming: verifies returning correct claimed amount after a partial claim milestone (test_get_vesting_claimed_returns_correct_amount_after_claiming).
• Multiple claims accumulation: verifies claimed amount accumulates accurately across multiple milestones up to 100% full vesting (test_get_vesting_claimed_accumulates_across_multiple_claims).

Verification

• cargo test --test token_tests test_get_vesting_claimed: 4/4 tests pass.
• Adheres strictly to project coding standards and test conventions.