This PR addresses an issue where, if an operation's input includes an event streaming member, the builder for the operation input or output may raise a ConstraintViolation when build_enforcing_all_constraints is called and the event streaming member field is not set. This occurs because the member shape is required.

The standard error message that is shown when ValidationException is not attached to an operation is also displayed in this case:

Operation test#TestOperation takes in input that is constrained (https://awslabs.github.io/smithy/2.0/spec/constraint-traits.html), and as such can fail with a validation exception. You must model this behavior in the operation shape in your model file.

use smithy.framework#ValidationException

operation TestOperation {
    ...
    errors: [..., ValidationException] // <-- Add this.
}

Closes: 3813