Fix `Length::UpTo` usage in `FsBuilder` so it does not exceed available file length (#3797)

## Motivation and Context
https://github.com/awslabs/aws-sdk-rust/issues/821

## Description
This PR addresses an issue with the
[Length::UpTo](https://docs.rs/aws-smithy-types/1.2.2/aws_smithy_types/byte_stream/enum.Length.html)
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](https://github.com/awslabs/aws-sdk-rust/issues/821#issuecomment-1937354372)
(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](https://github.com/awslabs/aws-sdk-rust/issues/821#issuecomment-1937354372)
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
- [x] 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.
- [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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