Fix: validate body contents for empty operation input across protocols(issue 3723)

[lee-1104]

Hi, this is my first contribution to smithy-rs. Happy to address any feedback!

Motivation and Context

fixes #3723

When an operation has empty or no modeled input, the server was not validating the request body contents. This allowed malformed requests to be silently accepted.

Description

Added body validation in ServerHttpBoundProtocolGenerator for operations with empty or no input:

• JSON protocols (RestJson, AwsJson): accepts empty body or {}
• CBOR (RpcV2Cbor): accepts empty body or empty CBOR map (0xA0)
• XML (RestXml): accepts only empty body
• Removed AdditionalTokensEmptyStruct from the failing test list since it now passes.

Testing

• cargo test additional_tokens_empty_struct passes in rpcv2Cbor_extras
• ./gradlew codegen-server-test:assemble completes successfully

Checklist

• 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.

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