fix: remove invalid #[serde(flatten)] from branch deserialization test (#49)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bashandbone <89049923+bashandbone@users.noreply.github.com>

Author: Copilot

src/options.rs

@@ -716,7 +716,7 @@ mod tests {
         // SerializableBranch appears as a field value in real config files.
         #[derive(Debug, serde::Deserialize)]
         struct Helper {
-            #[serde(flatten)]
+            #[expect(dead_code)]
             branch: SerializableBranch,
         }