refactor(test): slice the bytes, not the str (sliced_string_as_bytes)

`line[..s].as_bytes()` re-walks UTF-8 boundaries to slice the &str
first; `&line.as_bytes()[..s]` slices the byte view directly. `s` is a
byte offset from `str::find`, so the two are equivalent here. Clears
the last clippy warning in the workspace.

crates/ailang-core/tests/design_index_pin.rs

@@ -229,7 +229,7 @@ fn contracts_carry_no_decision_record_prose() {
         if line.contains("**Status: ") { return Some("**Status:"); }
         if line.contains("21.g") { return Some("21.g"); }
         if let Some(s) = line.find(" sketch") {
-            let pre = line[..s].as_bytes();
+            let pre = &line.as_bytes()[..s];
             if pre.last().is_some_and(u8::is_ascii_alphanumeric)
                 && pre.iter().rev()
                       .take_while(|c| c.is_ascii_alphanumeric())