Fix Length::UpTo usage in FsBuilder so it does not exceed available file length

[ysaito1001]

Motivation and Context

awslabs/aws-sdk-rust#821

Description

This PR addresses an issue with the Length::UpTo usage in FsBuilder. Previously, if a value specified for UpTo exceeded the remaining file length, FsBuilder would incorrectly accept the value. This discrepancy led to failures in subsequent request dispatches, as the actual body size did not match the advertised Content-Length, as explained here (thank you @pablosichert for a self-contained reproducer and problem analysis!).

Testing

• Added a unit test for FsBuilder verifying the Length::UpTo usage
• Ran successfully a customer provided reproducer with the code changes in this PR (with an added a call to complete_multipart_upload at the end, it also succeeded in uploading the object):

upload_id: cTDSngbubD25cOoFCNgjpG55o0hAMQNjO16dNFyNTKjg9PEtkcrKG5rTGzBns7CXoO8T.Qm9GpNj6jgwJTKcXDpsca95wSMWMDfPF0DBhmbk3OAGHuuGM1E70spk2suW
File created
part_number: 1, e_tag: "5648ddf58c7c90a788d7f16717a61b08"
part_number: 2, e_tag: "a6bdad6d65d18d842ef1d57ca4673bc3"
part_number: 3, e_tag: "f518f6b19b255ec49b61d511288554fc"
part_number: 4, e_tag: "1496524801eb1d0a7cfbe608eb037d9c"
part_number: 5, e_tag: "21340de04927ce1bed58ad0375c03e01"

Checklist

• For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "client," "server," or both in the applies_to key.
• 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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[github-actions]

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• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• Server Test Python (ignoring whitespace)
• No codegen difference in the Server Test Typescript

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[Velfi]

Thanks for fixing this Yuki