Test: payment below min_amount is rejected

[phertyameen]

Issue #67: Test: payment below min_amount is rejected

Category: Testing
Repo: bridgelet-core (Soroban / Rust)

Description

Write a test passing a payment amount below the minimum set during initialize() and verify Error::PaymentBelowMinimum is returned.

Acceptance Criteria

• Rust implementation complete
• cargo test passing
• Integration test on local Stellar sandbox passing
• Docs updated (docs/ or inline rustdoc)
• Reviewed and merged