Fix protocol selection behavior in `ClientProtocolLoader` (#4165)

## Description
The bugs include
- The default SDK-supported protocols (`DefaultProtocols`) were listed
in an incorrect priority order.
- Protocol resolution logic incorrectly iterated over service-applied
protocols, which is returned by `getProtocols()` whose result may not
reflect the intended priority.

This PR addresses these issues.

## Testing
- Existing CI
- `ClientProtocolLoaderTest.kt`

## 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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