openapi: look up service comments by declaration index - #7600

References to other Issues or PRs

Refs #746 (the case that still happens on main; the two-service example in the issue already works).

Have you read the Contributing Guidelines?

Yes.

Brief description of what is fixed or changed

The registry leaves services without methods out of File.Services, but the source code info paths that hold comments use the index of the service declaration in the .proto file. Both generators used the File.Services index, so a service declared after an empty one reads the comments of the declaration before it:

// Placeholder service.
service Placeholder {}

// Manages books.
service BookService {
  // Gets a book.
  rpc GetBook(Book) returns (Book) {
    option (google.api.http) = {get: "/v1/book"};
  }
}

Here the BookService tag gets the description "Placeholder service." and GetBook has no summary. With Empty, A, B, B's tag and operation get A's comments.

The index is now looked up in the file's service declarations (File.GetService()): in protoc-gen-openapiv2 through findServiceIndex, which replaces the svcBaseIdx bookkeeping in renderServices, and in protoc-gen-openapiv3 through a serviceIndex helper used by serviceComments and methodComments.

Other comments

Added TestGenerateCommentsAfterEmptyService (v2) and TestServiceAndMethodCommentsAfterEmptyService (v3); both fail without the change.

I used Claude Code to write this change.