fix: generate requestBody for messages with oneOf and path params

[vitorbari]

Summary

Fix missing requestBody in generated OpenAPI specs for RPCs that use body: "*" with path parameters when the request message contains oneOf fields.

Problem

When a proto message has both regular fields and oneOf fields, MessageToSchema() restructures the schema by wrapping Properties and OneOf under an AllOf and setting Properties to nil:

  // schema.go lines 97-104
  if len(s.OneOf) > 0 && s.Properties.Len() > 0 {
      s.AllOf = append(s.AllOf,
          base.CreateSchemaProxy(&base.Schema{Properties: s.Properties}),
          base.CreateSchemaProxy(&base.Schema{OneOf: s.OneOf}),
      )
      s.Properties = nil
      s.OneOf = nil
  }

The body: "*" handling in paths.go then checks s.Properties != nil before generating the requestBody. Since Properties was moved into AllOf, this check fails and the entire request body is silently dropped.

For example, this proto:

  message UpdateRequest {
    string id = 1;       // path param
    string name = 2;     // body field
    oneof details {      // body oneOf
      Profile profile = 3;
      Settings settings = 4;
    }
  }

  rpc Update(UpdateRequest) returns (google.protobuf.Empty) {
    option (google.api.http) = {
      patch: "/resources/{id}"
      body: "*"
    };
  }

Previously generated an operation with no requestBody at all. After this fix it correctly generates a body with name and the profile/settings oneOf.

Test

Added a path_params test scenario with an UpdateRequest message that combines a path parameter, a regular field, and a oneOf group