Cherry-pick codegen fix for unions with the `@httpPayload` trait (#2987)

## Motivation and Context
Cherry-picks https://github.com/awslabs/smithy-rs/pull/2969

, since our
internal build based on the `smithy-rs-release-0.56.x` branch failed to
code gen for the latest `medicalimaging` SDK model.

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Co-authored-by: John DiSanti <jdisanti@amazon.com>