Implement GitHub Actions CI pipeline for Soroban contracts

[Code-Paragon]

Problem

There is no automated pipeline to enforce code formatting, run the comprehensive Rust test suite, or verify successful WASM compilation for incoming pull requests.

Expected behavior

A .github/workflows/rust-ci.yml file is created to automatically run make fmt, cargo clippy, and make test on every PR targeting the main branch.

Files to update

• .github/workflows/rust-ci.yml (create)

Project relevance

This guarantees that all open-source contributions adhere to the project's strict DeFi testing and compilation standards before maintainers review them.

Acceptance criteria

• Feature works as described
• Tests are added or updated where needed
• Documentation is updated where needed
• Pipeline successfully configures the wasm32-unknown-unknown target before building
• No TypeScript, lint, test, or build errors