test: cover farming pool lock credit edges

[jamilahmadzai]

Closes #21

Bounty: GrantFox OSS campaign for issue #21

Summary

• add exact balance assertions for lock and full/partial unlock paths
• assert the locked and unlocked farming-pool event payloads, filtering out SAC transfer events
• cover credit continuity across multiple lock windows so later deposits only accrue for later ledgers
• align the partial unlock test with the 500 locked / 200 unlocked / 300 remaining case from the issue

Validation

• cargo fmt --all -- --check
• cargo build
• cargo test -p farming-pool -- --nocapture
• cargo test --workspace -- --nocapture
• git diff --check

Notes

• test-only change; no contract runtime behavior changed
• Soroban still emits the existing Events::publish deprecation warnings from contract code