feat(pubsub): integrate publish hedging into AsyncPublisher and Service

[feywind]

Stack Structure (Stack #36644)

Layer	Branch	PR
1	hedging/options-and-token-bucket	#36639
2	hedging/coordinator-and-scheduler	#36640
3	hedging/queue-integration	#36641 (this PR)
4	hedging/queue-unit-tests	#36642
5	hedging/system-tests	#36643

Summary

Implements Layer 3 of Cloud Pub/Sub Publish Hedging (go/sdk:pubsub-hedging) for google-cloud-pubsub:

• TopicAdmin::CancellableRetryPolicy & TopicAdmin::Client operation hook (lib/google/cloud/pubsub/admin_clients.rb):
  • Registers active gRPC Operation instances with the thread-bound AttemptHandle so losing attempts can be cancelled mid-flight via Operation#cancel.
  • Prevents Gapic::ServiceStub::RpcCall from retrying GRPC::Cancelled when an attempt was intentionally cancelled, and uses AttemptHandle#sleep_interruptibly during retry backoff so attempt 0 wakes immediately on sibling completion.
• Service#publish & Service#publish_timeout (lib/google/cloud/pubsub/service.rb):
  • Adds call_options: and attempt_handle: support to Service#publish while preserving 1-arg publish_internal(request) compatibility when options are omitted.
• AsyncPublisher & Publisher integration (lib/google/cloud/pubsub/async_publisher.rb, lib/google/cloud/pubsub/publisher.rb):
  • Wires hedging: configuration into AsyncPublisher#initialize and Publisher, enforces mutual exclusion in enable_message_ordering!, dispatches unordered batches through CancellationSharer and HedgingScheduler, and tears down hedging resources cleanly in wait!.