fix hyper 1.x connection refused errors not marked as retryable (#4190)

## Description
internal ref: `V1820425457`

Fixes a regression in the way connection issues are classified which was
causing them to not be retried when using the hyper 1.x client.

The issue is the logic for [classifying the
error](https://github.com/smithy-lang/smithy-rs/blob/release-2025-06-11/rust-runtime/aws-smithy-http-client/src/client.rs#L401)
was never being hit because connection errors are no longer
`hyper::Error` they are `hyper_util::client::legacy::Error`.

*
[hyper-util::Error](https://docs.rs/hyper-util/0.1.14/hyper_util/client/legacy/struct.Error.html)
*
[hyper@1.x::Error](https://docs.rs/hyper/1.6.0/hyper/struct.Error.html)
*
[hyper@0.14.x::Error](https://docs.rs/hyper/0.14.32/hyper/struct.Error.html#method.is_connect

)

## Testing
New unit test plus verification against internal package that opened the
issue.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [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.

----

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---------

Co-authored-by: Landon James <lnj@amazon.com>