Extend ci cd for contracts

[laryhills]

⚙️ CI for Soroban contracts and Cargo setup

• Closes Extend CI Pipeline with Rust/Soroban Contract Checks and Update CONTRIBUTING.md #291

---

📌 Type of Change

• Added a contracts CI job (Rust stable, wasm32-unknown-unknown, rust-cache, cargo fmt, clippy, release WASM build, cargo test)
• Ignored .DS_Store and kept Rust artifacts ignored under contract/
• Documented contract prerequisites and local checks in CONTRIBUTING.md; noted contracts in required branch checks
• Declared soroban-sdk 23 and dev-deps with testutils

Files Changed:

• .github/workflows/ci.yml
• .gitignore
• CONTRIBUTING.md
• contract/Cargo.toml

[drips-wave]

@laryhills Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits