style(router): drop unnecessary crate:: prefix on RouterError matcher

`RouterError` is already imported at the top of the file, so the
two test-side `matches!(result, Err(crate::RouterError::UpstreamProtocol(_)))`
uses trip clippy's `unused_qualifications` under `-D warnings`. Drop
the prefix on both sites; functional behaviour is unchanged.

These warnings predate this PR (originated in 25abc9e on 2026-06-07)
but NVIDIA's `rust:lint` re-runs because this PR touches
`backend.rs`, so the lint regression surfaces here.

Signed-off-by: st-gr <38470677+st-gr@users.noreply.github.com>

Author: st-gr

crates/openshell-router/src/backend.rs

@@ -1036,7 +1036,7 @@ mod tests {
         .await;
 
         assert!(
-            matches!(result, Err(crate::RouterError::UpstreamProtocol(_))),
+            matches!(result, Err(RouterError::UpstreamProtocol(_))),
             "over-cap response must fail as UpstreamProtocol, got: {result:?}"
         );
     }
@@ -1081,7 +1081,7 @@ mod tests {
         );
         let result = super::read_capped_response_body(response, 8).await;
         assert!(
-            matches!(result, Err(crate::RouterError::UpstreamProtocol(_))),
+            matches!(result, Err(RouterError::UpstreamProtocol(_))),
             "over-cap chunked body must be rejected by the loop, got: {result:?}"
         );
     }