feat(pubsub): add HedgingOptions and fixed-point HedgingTokenBucket

[feywind]

Stack Structure (Stack #36644)

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

Summary

Implements Layer 1 of Cloud Pub/Sub Publish Hedging (go/sdk:pubsub-hedging) for google-cloud-pubsub:

• AsyncPublisher::HedgingOptions (lib/google/cloud/pubsub/async_publisher/hedging_options.rb):
  • Opt-in configuration with defaults (enabled: true, hedge_delay: 1.0, max_tokens: 50, refill_ratio: 0.1, plus delay alias).
  • Validates parameter bounds (0.1 <= hedge_delay <= 10.0, integer 1 <= max_tokens <= 250, 0.001 <= refill_ratio <= 0.2), mutual exclusion with message_ordering, and hedge_delay < publish_timeout.
• AsyncPublisher::HedgingTokenBucket (lib/google/cloud/pubsub/async_publisher/hedging_token_bucket.rb):
  • Thread-safe fixed-point token bucket rate limiter scaled by 1,000 (1.0 logical token = 1,000 scaled units) starting at 0 tokens on cold start.
• Unit tests (test/google/cloud/pubsub/async_publisher/hedging_token_bucket_test.rb):
  • Covers bounds/ordering/timeout validation, fixed-point fractional refills without floating-point drift, capping at max_tokens, and multi-threaded concurrent stress testing.