Fix CPU spinning when curl_easy_send returns 0 bytes on SSL socket writes

[harshitkr12]

Summary:

Why ?
When uploading blobs to Azure Storage over HTTPS (SSL/TLS), curl_easy_send() can return CURLE_OK with 0 bytes sent. This would lead to a retry without delay immediately, causing a tight CPU-spinning loop.

What ?
Added a 10ms sleep when curl_easy_send() returns CURLE_OK with 0 bytes sent to prevent rapid retries and allow the OS scheduler to run other threads.

Test Plan:

cmake --build .
ctest -R "azure-core" --output-on-failure
>> 100% tests passed, 0 tests failed out of 459

Fixes #6817

[github-actions]

Thank you for your contribution @harshitkr12! We will review the pull request and get back to you soon.

[antkmsft]

Thank you! I am approving the pull request, it looks good to me. I'd like @LarryOsterman to review before merging.

@LarryOsterman, if you will be doing the next release (it is probably January 8, unless @ronniegeraghty says it is a week after), please make sure that the changelog has an entry for this fix, as well as the acknowledgements section + credit after the fix line, similar to the other fixes that we've taken.
The upcoming release should probably be the beta.

OR, I think we can wait for the week of Jan 12th - it is still going to be a release week, and then I can make the release myself. I suggest we do that.

But just in case, here is the hange that we'll need for the CHANGELOG.md in core:

# Release History

## 1.17.0-beta.1 (2026-01-08)

### Bugs Fixed

- [[#6817]] Fix CPU spinning when curl_easy_send returns 0 bytes on SSL socket writes. (A community contribution, courtesy of _[sushshring](https://github.com/harshitkr12)_)

### Acknowledgments

Thank you to our developer community members who helped to make Azure Core better with their contributions to this release:

- harshitkr12 _([GitHub](https://github.com/harshitkr12))_

I am going to follow up with Rubrik in the issue letting them know how they can start consuming it right away, if they need it.

[antkmsft]

/azp run cpp - core

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[antkmsft]

Live tests are green.

[antkmsft]

/azp run cpp - storage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[RickWinter]

std::this_thread::yield will give up the time slice to other like or higher priority threads. Does curl actually need the thread to suspend (sleep) in case?