fix: update librarian to v0.39.1-0.20260831182628-509cd77c2487

[suztomo]

Updates Librarian to v0.39.1-0.20260831182628-509cd77c2487 (incorporating googleapis/librarian#7455 to respect all: rest in sdk.yaml) and regenerates all libraries with librarian generate --all.

This update respects REST-only transport configurations in sdk.yaml by passing only --ruby_out to protoc and omitting --grpc_out and --plugin=protoc-gen-grpc, removing unnecessary *_services_pb.rb stub files from pure REST libraries.

Note that the discrepancy between transports: ... rest configurations and generating/committing *_services_pb.rb files had already started before Librarian migration under OwlBot generation. For example, lib/google/ads/admanager/v1/ad_break_service_services_pb.rb in google-ads-ad_manager-v1 was originally introduced in commit ef7575eaf7f6cf8d975c0f7af2040fe85b0e4cf0 (Date: Tue Jul 8 14:24:06 2025 -0700, PR #30579) by OwlBot, even though Ad Manager is a pure REST client.

Finding in Legacy Bazel Files

In the legacy Bazel pipeline:

• ruby_cloud_gapic_library in BUILD.bazel correctly had transport = "rest", so gapic-generator-ruby only generated REST GAPIC client code.
• However, the packaging rule ruby_gapic_assembly_pkg unconditionally listed ruby_grpc_library (:admanager_ruby_grpc) in its deps.
• _ruby_gapic_assembly_pkg_impl in rules_ruby_gapic/ruby_gapic_pkg.bzl unpacked all deps (including gRPC stubs from ruby_grpc_library) into the output archive /lib directory.
• OwlBot then copied this assembled package into google-cloud-ruby.

SemVer / Commit Prefix

The removal of these unused stub classes is tentatively set to fix: (no breaking change) because the underlying services have not supported gRPC and the generated GAPIC clients for these libraries have always operated over REST only.

Affected Gems

• google-ads-ad_manager-v1: Removed 62 obsolete *_services_pb.rb service stubs.
• google-cloud-api_hub-v1: Removed 9 obsolete *_services_pb.rb service stubs.
• google-cloud-ftp-v1: Removed 1 obsolete *_services_pb.rb service stub.
• google-cloud-gke_connect-gateway-v1: Removed 1 obsolete *_services_pb.rb service stub.
• google-cloud-gke_connect-gateway-v1beta1: Removed 1 obsolete *_services_pb.rb service stub.

Verification

Local CI checks passed with 0 offenses and 0 failures:

toys ci --test --rubocop --gems google-ads-ad_manager-v1,google-cloud-api_hub-v1,google-cloud-ftp-v1,google-cloud-gke_connect-gateway-v1,google-cloud-gke_connect-gateway-v1beta1

This fix is a step towards googleapis/librarian#7452.

BEGIN_COMMIT_OVERRIDE
fix: update librarian to v0.39.1-0.20260831182628-509cd77c2487 to respect "all: rest"
END_COMMIT_OVERRIDE