feat(openapi): Remove go_package option requirement

[amckinney]

Problem

The protogen library requires every .proto file to have a go_package option, but this plugin generates OpenAPI specs — not Go code.

Solution

Instead of calling opts.Run(), we now do the following:

1. Read and unmarshal the CodeGeneratorRequest from stdin ourselves.
2. Inject a synthetic go_package option (example.com/fake/<package>) into any .proto file that's missing one — this satisfies protogen's validation without affecting the OpenAPI output at all.
3. Call opts.New(req) with the patched request, then handle the response ourselves.