Reset offsets in buffer list for retries

[ypatia]

There's a bug in the retry logic when a 503 is encountered while uploading data. Our curl client fails to reset its BufferList state back to zero in this case and we end up sending a bunch of garbage data to the cloud service.

This PR fixes the issue and adds a regression test that failed before and passes with the fix.

[sc-49128]

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TYPE: BUG
DESC: Reset offsets in buffer list for retries

[Shelnutt2]

Is there any changes needed related to buffer_list_seek_callback or CURLOPT_SEEKFUNCTION? We use this for the redirection which I believe is working fine but since it involves a similar type of reply of the buffers wanted to ask.

[ypatia]

Is there any changes needed related to buffer_list_seek_callback or CURLOPT_SEEKFUNCTION? We use this for the redirection which I believe is working fine but since it involves a similar type of reply of the buffers wanted to ask.

Thanks for bringing this to my attention, I think this change is generic and applies to any retry with data stored in a BufferList structure, so it'll work for seek same as it works for plain read memory cb.