Fix panic in DateTime::from_secs_f64 (#3806)

## Motivation and Context
- #3805 

## Description
Recover from floating point values extremely close to whole numbers by
incrementing the epoch seconds value.

## Testing
- proptest

## Checklist
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- [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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