fix(assets): flush tokio file after multipart upload to prevent truncated reads

tokio::fs::File::write_all returns as soon as data is copied to an
internal buffer and a blocking write task is spawned — it does NOT wait
for the blocking write to complete. When the File is dropped without
flushing, the last write may still be in-flight. A subsequent fs::read
can then see a truncated file.

This caused flaky E2E failures in the compositor-image-overlay upload
test: the image crate's into_dimensions() would fail with 'unexpected
end of file' because it was parsing a partially-written PNG.

The plugin upload handler in server/mod.rs already had this fix; apply
the same pattern to all asset upload functions (image, audio, font) in
assets.rs and plugin_assets.rs.

Signed-off-by: StreamKit Devin <devin@streamkit.dev>
Co-Authored-By: Claudio Costa <cstcld91@gmail.com>

Author: streamkit-devin

apps/skit/src/assets.rs

@@ -277,6 +277,14 @@ async fn write_upload_stream_to_disk(
         }
     }
 
+    // Flush pending writes — tokio::fs::File::write_all returns as soon as
+    // data is copied to an internal buffer and a blocking write is spawned,
+    // so the last write may still be in-flight when the File is dropped.
+    if let Err(e) = file.flush().await {
+        let _ = fs::remove_file(file_path).await;
+        return Err(AssetsError::IoError(format!("Failed to flush file: {e}")));
+    }
+
     // Create default license file (best-effort).
     let license_path = file_path.with_extension(format!("{extension}.license"));
     // REUSE-IgnoreStart
@@ -710,6 +718,14 @@ async fn write_image_upload_to_disk(
         }
     }
 
+    // Flush pending writes — tokio::fs::File::write_all returns as soon as
+    // data is copied to an internal buffer and a blocking write is spawned,
+    // so the last write may still be in-flight when the File is dropped.
+    if let Err(e) = file.flush().await {
+        let _ = fs::remove_file(file_path).await;
+        return Err(AssetsError::IoError(format!("Failed to flush file: {e}")));
+    }
+
     Ok(total_bytes)
 }
 
@@ -1253,6 +1269,14 @@ async fn write_font_upload_to_disk(
         }
     }
 
+    // Flush pending writes — tokio::fs::File::write_all returns as soon as
+    // data is copied to an internal buffer and a blocking write is spawned,
+    // so the last write may still be in-flight when the File is dropped.
+    if let Err(e) = file.flush().await {
+        let _ = fs::remove_file(file_path).await;
+        return Err(AssetsError::IoError(format!("Failed to flush file: {e}")));
+    }
+
     // Create default license file (best-effort).
     let license_path = file_path.with_extension(format!("{extension}.license"));
     // REUSE-IgnoreStart

apps/skit/src/plugin_assets.rs

@@ -836,6 +836,14 @@ async fn write_upload_to_disk(
         }
     }
 
+    // Flush pending writes — tokio::fs::File::write_all returns as soon as
+    // data is copied to an internal buffer and a blocking write is spawned,
+    // so the last write may still be in-flight when the File is dropped.
+    if let Err(e) = file.flush().await {
+        let _ = fs::remove_file(file_path).await;
+        return Err(PluginAssetError::IoError(format!("Failed to flush file: {e}")));
+    }
+
     Ok(total_bytes)
 }