Support OpenAPI schema annotations on enum types

OpenAPI v3 annotations currently support files, methods, messages, and message fields, but not protobuf enum types.

Because `protoc-gen-openapi` generates enum schemas inline, there is no simple way to attach enum-level schema metadata (e.g. `title`, `description`, `example`, or `default`). In our case, this prevents downstream documentation renderers from using those values when generating examples for enum-typed fields.

Start at the protoc-gen-openapi annotation handling and enum schema generation paths. Trace how annotations are supported for files, methods, messages, and message fields, then determine where enum-level metadata should enter the generated schema. Done means enum annotations can provide values such as title, description, example, and default for downstream documentation rendering.

Written by the indexing model from the issue text.