CI: pin rustfmt in rust-toolchain.toml so cargo fmt finds it

The test workflow failed at `cargo fmt --check` with "'cargo-fmt' is not
installed for the toolchain '1.96'". The rust-toolchain.toml channel pin
(1.96) is what rustup resolves when cargo fmt runs, but the rustfmt
component installed via the toolchain action's `components:` input landed
on a different toolchain resolution. Listing the component in the toml
makes it the single source of truth, so rustup ensures rustfmt is present
for the pinned toolchain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R3vgA3Q6PR9VQn8X5pLcMR

Author: claude

rust-toolchain.toml

@@ -1,3 +1,7 @@
 [toolchain]
 # 1.96+ required: oxc 0.137 uses stabilized `if let` guards.
 channel = "1.96"
+# Pin rustfmt here so any toolchain rustup resolves from this file (e.g. when
+# `cargo fmt` triggers an install) always has the component — CI's separate
+# component install can land on a different toolchain resolution otherwise.
+components = ["rustfmt"]