build(toolchain): switch Rust channel to stable (#4775)

* Change Rust toolchain channel to stable

Signed-off-by: houseme <housemecn@gmail.com>

* style: apply clippy --fix and cargo fix lint suggestions

Run `cargo clippy --fix --all-targets --all-features` and
`cargo fix --lib --all-targets` across the workspace, then resolve the
remaining warnings by hand:

- collapse needless borrows in `format!` args, prefer `?` over explicit
  early returns, and use `.values()` / `.flatten()` iterator adapters
- rewrite the `Md5` scan loop via `manual_flatten` and re-indent the
  `select!` macro body (rustfmt skips macro interiors)
- annotate the intentional dead-code `Md5` inherent methods (constructed
  only by the test factory) with `#[allow(dead_code)]`

Behavior is unchanged.

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Signed-off-by: houseme <housemecn@gmail.com>
Co-authored-by: heihutu <heihutu@gmail.com>

rust-toolchain.toml

@@ -13,5 +13,5 @@
 # limitations under the License.
 
 [toolchain]
-channel = "1.96"
+channel = "stable"
 components = ["rustfmt", "clippy", "rust-src", "rust-analyzer"]