fix: Ensure file is always closed when saving blobs to disk

[vtjl10]

• Summary: Add defer file.Close() in saveBlobDataToDisk (util/headerreader/blob_client.go) to guarantee file descriptor release on all paths.
• Behavior: No functional change to blob saving; ensures cleanup on marshal/write errors.
• Rationale: Prevents file descriptor leaks that can lead to “too many open files” under failure or high-load scenarios; improves robustness.