CI/CD: Add Smart Contract build and test to CI pipeline

[Kami-no-san]

Description

Add Rust/Soroban contract compilation and testing to the GitHub Actions CI pipeline.

Files to Modify

• .github/workflows/ci.yml — add contract build and test jobs

Acceptance Criteria

• Rust toolchain installed in CI (stable)
• wasm32v1-none target added
• All three contracts build successfully
• Contract unit tests run (cargo test)
• Contract build artifacts cached between runs
• Job fails if contracts don't compile
• Job fails if tests don't pass
• Contract linting with cargo clippy