Refactor fee computation and validation functions

[Biokes]

Integration Tests for Soroban Event Ingestion

Closes #24

Summary

Implements end-to-end integration tests for Soroban event ingestion, verifying that all contract events index correctly via the Stellar CLI.

Changes

• Added integration tests simulating full contract lifecycle to assert event emission and indexing
• Verified all relevant events are emitted with correct topics and data payloads
• Ensured tests run against a local Soroban sandbox via soroban-cli

Testing

• cargo fmt — passed
• cargo clippy — passed, no warnings
• soroban contract build — compiles to WASM within size budget
• All integration tests pass against local sandbox

Notes

• No new runtime dependencies introduced
• Tests are isolated and do not mutate shared state between runs
• Any architectural decisions made during implementation are documented inline

[meshackyaro]

Well done and thanks for your contribution.