fix: ensure file transfer service always writes uploaded file

[jarhodes314]

Proposed changes

Fixes some flaky tests and a bug in the file transfer service where files were not uploaded in their entirety.

We were previously calling .write, but ignoring the number of bytes written. Since we always want to write the entire chunk, I have replaced this with .write_all. In addition, I've added a BufWriter to improve the performance when writing files, and ensured this buffer is flushed before responding to the request.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

I've seen multiple failures locally, all of them exhibit the same "empty file" behaviour, and cannot reproduce any tedge-agent failures after applying the fix:

• flaky unittest: file_transfer_server::http_rest::tests::uploaded_file_can_be_downloaded_from_the_api #3030
• flaky test: file_transfer_server::actor::tests::http_server_put_and_get #2816

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../core/tedge_agent/src/http_server/file_transfer.rs	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

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[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
556	0	2	556	100	1h34m53.377021s

[albinsuresh]

LGTM

[didier-wenzek]

Approved