Fix server code generation of `@httpPayload`-bound constrained shapes (#2584)

The issue is we're not changing the return type of the payload
deserializing function to be the unconstrained type (e.g. the builder in
case of an `@httpPayload`-bound structure shape) when the shape is
constrained.

Yet another example of why code-generating `constraints.smithy` (see
#2101)
is important.

Closes #2583.

## Testing

The added integration test operation fails to compile without this
patch.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

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