Update OperationInputTestDecorator to find operation shape by name (#2782)

## Motivation and Context
Addresses https://github.com/awslabs/smithy-rs/issues/2767



This fixes an issue for users who write endpoint tests that rely on an
operation and service shape from different namespaces.

## Description
Instead of assuming operation namespace matches service namespace, 

## Testing
`./gradlew :aws:sdk-codegen:test`


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Co-authored-by: Zelda Hessler <zhessler@amazon.com>