Task: Integration test for the protoc-gen bin

Do we test the protocol-gen-twirp_ruby ?
1. I see the go bin has a main_test.go file, but I don't see we call it during our action CI build
Note: `cd protoc-gen-twirp_ruby && go test ./...` will execute the test
2. We could have a integration test, where from a .proto file we generate the stubs, and asserts against that, to prevent major breaking changes.
3. Idea: bring some of the example app as part of our test suite.

1 is a easy win, 2 and 3 are more like ideas, but the overall goal here is to have tests that trigger the protoc ruby bin.

Start in protoc-gen-twirp_ruby and inspect main_test.go, then run `cd protoc-gen-twirp_ruby && go test ./...` to check whether the existing tests run in CI. Review the action CI configuration and the .proto generation flow; done means the protoc Ruby binary is exercised by automated tests, with generated stubs or an example app covered if that scope is chosen.

Written by the indexing model from the issue text.