fix: fix EntityTooSmall issue

[WenyXu]

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What's changed and what's your intention?

We rely on the BufferedWriter to avoid writing too small entities to S3 directly. However, the into_futures_async_write will break this feature(it introduces another buffer in the underlying writer). And consider simplifying the BufferredWriter in the future.

1. fix S3 returns EntityTooSmall error issues
2. Add a unit test for this case.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.18%. Comparing base (4e5dd1e) to head (e1259b2).
Report is 2 commits behind head on main.

❗ Current head e1259b2 differs from pull request most recent head 1a28c48

Please upload reports for the commit 1a28c48 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4100      +/-   ##
==========================================
- Coverage   85.47%   85.18%   -0.30%     
==========================================
  Files         991      991              
  Lines      173053   173151      +98     
==========================================
- Hits       147920   147491     -429     
- Misses      25133    25660     +527     

[v0y4g3r]

LGTM

[v0y4g3r]

Maybe elaborate the reason why the entity will be 256KB if chunk parameter is not provided in the pr detail.