fix: drop a redundant borrow clippy now rejects

error: redundant reference in `assert_eq!` argument
      --> tests/injection.rs:633:13
      = note: `-D clippy::useless-borrows-in-formatting` implied by `-D warnings`

No code changed to cause this. ci.yml installs whatever rustup calls
stable and the tree pins no toolchain, so the gate moved under the
repository: `useless_borrows_in_formatting` fires on a line that has been
there since the injection tests were written, and `RUSTFLAGS: -D
warnings` turns it into a hard failure.

The borrow was never doing anything -- `{:?}` formats a String and a
&String identically -- so this is the lint being right, not a workaround
for it.

Verified against the toolchain CI actually resolved to (1.97.1, not the
1.85 in Cargo.toml's rust-version), all four gates the workflow runs:
fmt clean, clippy --all-targets --all-features clean with no other lint,
208 tests passing, and cargo deny reporting advisories/bans/licenses/
sources ok.

Worth separating from this fix: the failure was invisible until it
happened because the toolchain floats. Nothing here pins it, so the next
stable release can turn this repository red without a commit. A
rust-toolchain.toml would close that, at the cost of not seeing new lints
until it is bumped -- a policy call, not a bug fix, so it is not in here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/injection.rs

@@ -630,7 +630,7 @@ async fn the_audio_signature_field_cannot_carry_arbitrary_bytes() {
             status,
             StatusCode::BAD_REQUEST,
             "signature {:?} should be rejected: {body}",
-            &sig.chars().take(40).collect::<String>()
+            sig.chars().take(40).collect::<String>()
         );
     }
 }