Fix Rust 1.82 lint warnings in client runtime / SDK crates (#4126)

## Motivation and Context
Address warnings in client runtime / SDK crates as listed in #4122 

## Description
`TODO(MSRV1.82 follow-up)` are retained, since warnings coming from the
server runtime crates need to be addressed as well.

## Testing
No lint warnings are generated with the changes.

```
aws-config git:(ysaito/fix-msrv-1-82-lint-warning) ✗ RUSTFLAGS="-Dwarnings" cargo clippy --all-features
    ...
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 35.80s
```
```
aws-sigv4 git:(ysaito/fix-msrv-1-82-lint-warning) ✗ RUSTFLAGS="-Dwarnings" cargo clippy --all-features
    ...
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 41.40s
```

```
s3 git:(ysaito/fix-msrv-1-82-lint-warning) ✗ RUSTDOCFLAGS='--cfg docsrs -Dwarnings' cargo +nightly-2025-05-04 doc --no-deps --document-private-items --all-features
    ...
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 16.16s
```
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