perf(next-napi): optimize output assets collection using flat_map (#81222)

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Replace manual loop with `extend()` calls with a single `flat_map().collect()` operation to reduce function call overhead and improve memory allocation efficiency when collecting output assets from multiple endpoints.

Author: fireairforce

crates/napi/src/next_api/project.rs

@@ -956,10 +956,10 @@ async fn output_assets_operation(
         .try_join()
         .await?;
 
-    let mut output_assets: FxIndexSet<ResolvedVc<Box<dyn OutputAsset>>> = FxIndexSet::default();
-    for assets in endpoint_assets {
-        output_assets.extend(assets.iter());
-    }
+    let output_assets: FxIndexSet<ResolvedVc<Box<dyn OutputAsset>>> = endpoint_assets
+        .iter()
+        .flat_map(|assets| assets.iter().copied())
+        .collect();
 
     Ok(Vc::cell(output_assets.into_iter().collect()))
 }