ci(contracts): ensure rustfmt is installed so the fmt gate actually runs

[broda-spendy]

Closes #1302

The rust-test job in rust-wasm.yml already runs cargo fmt --all -- --check, but installs the toolchain with profile minimal plus only the clippy component, so cargo fmt fails with missing-component instead of gating on formatting. This adds rustfmt to the installed components and applies the outstanding cargo fmt reformatting to bridge-compat (whitespace-only, produced by cargo fmt itself).

Deliberately out of scope (covered by sibling PRs, verified not duplicated via git log and gh pr list):

• PR fix(contracts): restore compilation across all four contract crates #1416 (fix/1303-rust-merge-corruption) restores vault compilation, including the lib.rs delimiter mismatch that still fails cargo fmt --all --check on this branch.
• PR ci(contracts): type-check every contract crate before merge #1417 (fix/1304-enforce-cargo-check) adds cargo check --workspace --all-targets --locked.

Verification (Windows, cargo +stable-x86_64-pc-windows-gnu, GNU toolchain):

• Before (upstream/main): cargo fmt --all -- --check exits 1 with bridge-compat drift plus vault lib.rs:1375 mismatched-delimiter parse error.
• After: cargo fmt -p bridge-compat -- --check exits 0; cargo fmt --all -- --check now fails only on the vault parse error owned by fix(contracts): restore compilation across all four contract crates #1416.
• No overlap: git diff -w on bridge-compat shows layout-only changes; rust-wasm.yml diff is the single components line plus comment.

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