fix(litellm): gate tests on server feature to fix bare cargo test

The litellm test module uses server-only structs (ActivityModelEntry,
SpendModelEntry) that are behind #[cfg(feature = "server")]. Gate the
test module with #[cfg(all(test, feature = "server"))] so bare
cargo test (without --features server) compiles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/infrastructure/litellm.rs

@@ -292,7 +292,7 @@ pub async fn get_litellm_usage(
     })
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "server"))]
 mod tests {
     use super::*;