Operations with an event stream member shape must have `ValidationException` (#3814)

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.*

```smithy
use smithy.framework#ValidationException

operation TestOperation {
    ...
    errors: [..., ValidationException] // <-- Add this.
}
```
Closes: [3813](https://github.com/smithy-lang/smithy-rs/issues/3813

)

---------

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>