fix(amazon): flush file buffer in S3Hook.download_file() before returning path

[pippo995]

Summary

• S3Hook.download_file() writes S3 object content to a file via download_fileobj() but never calls flush() before returning the file path
• When the caller immediately reads the returned path, the file may contain 0 bytes because data is still in Python's write buffer
• Added file.flush() after download_fileobj() to ensure buffered content is written to disk

Details

The original implementation used a with context manager which auto-closes (and flushes) the file. When preserve_file_name support was added, the with was removed and the file is now left open and unflushed.

This particularly affects small files (< ~8KB) that fit entirely in the buffer. The bug is latent in all environments but was exposed by apache-airflow-providers-common-compat==1.13.1 (PR #61157), which changed the execution timing of get_hook_lineage_collector() between download_fileobj() and return file.name.

[boring-cyborg]

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[vincbeck]

Tests are failing

[pippo995]

The tests were mocking NamedTemporaryFile to return a PosixPath instead of a file-like object. PosixPath has no .flush() and its .name returns just the filename (not the full path like a real file object). I'm going to change tests

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.