Update XML serialization for unions to properly handle unit struct members (#4306)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Fixing two CI issues that recently popped up. See
https://github.com/smithy-lang/smithy-rs/actions/runs/17771171675/job/50511115119#step:4:1340
for a CI run that exhibited both failures.

### Codegen Bug

Fixes a bug in `XmlBindingTraitSerializerGenerator.kt` exposed by a
change to the [S3-Control model
](https://github.com/awslabs/aws-sdk-rust/commit/effdaf98b3eb49e8900838176c18f72db48644ff#diff-101f9ff3d2e67a2122db6bc609ccf4baffc6aee548757139badd8d1c41ab11ab).
The model change introduced a smithy union type that contained a unit
struct as a member. This generated a match arm like:
```
crate::types::ObjectEncryptionFilter::Sses3(_inner) =>
{
    scope_writer.start_el("SSE-S3").finish();
}
```

`inner` is never used since there is nothing to serialize for a unit
struct, so this caused a [warning and CI
failure](https://github.com/smithy-lang/smithy-rs/actions/runs/17771171675/job/50511115119#step:4:1340).
The change here fixes that warning by prefixing `_inner` in these cases.

Some other serializer generators also appear to have this bug. Created
https://github.com/smithy-lang/smithy-rs/issues/4308

 to track.

### Incompatible lambda-http Version Bug
To unblock CI, this PR also bumps `lambda_http` to 0.8.4 in
`aws-smithy-http-server`.

## Testing
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<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Failed CI tasks are now passing


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Co-authored-by: ysaito1001 <awsaito@amazon.com>