test(pubsub): add live acceptance tests for publish hedging

[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
4	hedging/queue-unit-tests	#36642
5	hedging/system-tests	#36643 (this PR)

Summary

Implements Layer 5 of Cloud Pub/Sub Publish Hedging (go/sdk:pubsub-hedging) for google-cloud-pubsub:

• Live acceptance test suite (acceptance/pubsub/hedging_test.rb):
  • End-to-end delivery with custom hedging settings and token bucket capping at max_tokens.
  • Hedged attempt 1 wins against live Cloud Pub/Sub while primary attempt 0 is stalled and cancelled.
  • Primary attempt 0 wins after hedge dispatch and cancels in-flight hedged attempt 1.
  • Rate limiting when the token bucket has fewer than 1.0 token (< 1,000 scaled units).
  • Mutual exclusion with enable_message_ordering!.