Add `cargo fmt --check` job to contracts CI

[codebestia]

Add a dedicated fmt job to .github/workflows/contracts-ci.yml that runs cargo fmt --all -- --check across all packages in the workspace. No caching needed. Run cargo fmt --all locally and commit changes before merging so the baseline is clean.

Acceptance criteria:

• A PR with unformatted Rust code in any contract fails the fmt job
• The fmt job completes in under 30 seconds
• Existing code passes the check on merge

[karanjakevin39-collab]

@karanjakevin39-collab has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer , can i work on this issue ?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @karanjakevin39-collab to this issue.

[drips-wave]

Congratulations, @thefifthdev! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @thefifthdev: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊