Remove Violations usages from test harness

[jchadwick-buf]

Related to #96.

This should be released separately from (and before) #261, where we can then update the module dependency for protovalidate-testing. That'll avoid the need for tricky business with module syncing.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Oct 16, 2024, 2:45 PM

[jchadwick-buf]

CI failure is completely unrelated, looks like Bazel broke stuff. Guess I'll need to look into that.

[jchadwick-buf]

Going slightly bolder, I think we should just remove Violation from the public API altogether. Although I can see the benefit to using Protobuf to define API details across different languages, we now want to expose error details that can't really be expressed in protobuf (essentially live pointers) so the benefit is diminished. Someone may wish to embed the Violation message, but I don't really see why it's any different if they just define their own Violation message versus just using ours. So moving Violation to harness and removing it from the main validate.proto.

[jchadwick-buf]

Unfortunately this is not as clear cut as I thought. What I did not consider is that Violations is already in use as an error details message. So while we may still want to consider removing it for 1.0, we at least need to do something about some of those existing uses (e.g. in connectrpc/validate-go) so I think now is just not the right time to bite that one off.