Ensure request body availability after openapi3 filter validation

[ikattey]

This PR addresses an issue originally reported in the kin-openapi project (getkin/kin-openapi#743), where the request body was not available to handlers after being processed by the openapi3 filter.

This change:

• makes a copy of the original request body before validation.
• resets it afterwards using the copy, ensuring it's available for subsequent processing.

Existing tests cover this functionality but don't reproduce the issue. It's also not clear why the issue occurs, as openapi3filter.ValidateRequest() appears to reset the request body.

In any case, this approach makes request handling more robust – regardless of how the openapi3 filter or any future updates to it might handle the body.

[jamietanna]

Thanks - will try and have a look soon, and then make sure that this is consistent across all middlewares :)