[Infra] Add cargo fmt --check and clippy gates to contracts CI

[ogazboiz]

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Why this matters

The contracts CI job (.github/workflows/ci.yml) builds the WASM and runs cargo test, but there is no formatting or lint gate for the Rust code — no cargo fmt --check and no cargo clippy. The repo has no rustfmt.toml. This means style drift and common Rust foot-guns (needless clones, unwraps, etc.) land unflagged, and contributors get no fast local signal. Adding these two checks is cheap and catches a whole class of review nits automatically.

Acceptance criteria

• Add a cargo fmt --all -- --check step to the contracts job in .github/workflows/ci.yml (fails the build on unformatted code).
• Add a cargo clippy --all-targets --target wasm32-unknown-unknown -- -D warnings step (or a sensible warning gate appropriate for #![no_std] Soroban code).
• Ensure the existing contracts/ source passes both checks (apply cargo fmt and fix or #[allow] justified clippy findings).
• Add rustfmt and clippy components to the toolchain setup step.

Files to touch

• .github/workflows/ci.yml
• contracts/ (formatting fixes as needed)

Out of scope

• JS/TS lint changes (frontend already runs npm run lint).

[Signor1]

Can I work on this ?

[emdevelopa]

@emdevelopa has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hiii maintainer i'd like to jump on this issue

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