test: add zero cliff period tests for forge-vesting — closes #80

[spiffamani]

Summary

Adds tests for forge-vesting with cliff_seconds = 0, closing issue #80.

Changes

• Fixed setup_with_token() to correctly extract Address from StellarAssetContract
• Fixed balance checks to use token::Client instead of StellarAssetClient
• Fixed pre-existing cancel tests to use setup_with_token() for real token transfers
• Added 6 zero cliff period tests

Zero Cliff Tests Added

• test_zero_cliff_initialize_succeeds — cliff=0 is valid config
• test_zero_cliff_claim_succeeds_immediately — claim works after 100s with no cliff
• test_zero_cliff_correct_vested_amount_at_halfway — 500_000 vested at t=500
• test_zero_cliff_fully_vested_after_duration — fully vested at t=2000
• test_zero_cliff_claim_immediately_after_initialize — NothingToClaim at t=0
• test_zero_cliff_vesting_starts_immediately — 1_000 vested at t=1

Test Results

test result: ok. 19 passed; 0 failed; 0 ignored

Checklist

• cargo test -p forge-vesting — 19/19 passing
• No unsafe code
• No new dependencies

[spiffamani]

please check and merge my job all my test passed, base on the job you recomended