fix(python): constrain multipart upload size to fixed length

[abhiaagarwal]

Description

Object stores expected fixed lengths for all multipart upload parts right up until the last part. The original logic just flushed when it exceeded the threshold. Now, it flushes when the threshold is met exclusively with the same fixed buffer, unless we're completing the transaction, in which case the last piece is allowed to be smaller.

Bumps the constant to reflect that the minimum expected size by most object stores is 5MiB. Also adds a UserWarning if a constant is specified to be less.

Also releases the GIL in more places by moving the flushing logic to a free function.

Related Issue(s)

Closes #2605

Documentation

See: MultipartUpload docs

[github-actions[bot]]

ACTION NEEDED

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[ion-elgreco]

@abhiaagarwal looks good, just one remark on the buffer size warning

[abhiaagarwal]

@ion-elgreco don't merge yet — I found about an edge case in my logic that won't work (what happens when remaining = max_buffer_length = len(chunk)?) and I want to fix up the test to actually parameterize it + check for the warning message. I also opened an upstream issue in object_store to pull out the "fixed buffer" logic into its own struct so it can live upstream, for the future :)

[rtyler]

marking this as a draft because that also helps me of void hitting that big tempting green button