Fix presigned put bug with flex checksums (#3971)

## Motivation and Context
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Fixing bug reported in
https://github.com/awslabs/aws-sdk-rust/issues/1240#issuecomment-2594822919

## Description
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## Testing
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to -->
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Updated presigning integ tests to account for behavior specified in SEP,
added new test for the user provided checksum case

## Checklist
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rather than leaving it unchecked -->
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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