feat(rpm): add token bucket smoothing for RPM rate limiting
- New RPMTokenBucketService: per-account continuous-refill token buckets
(rate = rpm/60 tokens/sec, capacity = rpm). No new dependencies.
- GatewayService.AcquireRPMToken() delegates to the bucket service.
- Gateway handler inserts RPM token wait BEFORE wrapReleaseOnDone in both
Gemini and Anthropic dispatch paths; timeout returns 429 and releases slot.
- Config: gateway.rpm_smoothing.enabled (default false) + max_wait_ms (default 5000).
- 7 unit tests covering: immediate acquire, zero RPM, timeout, wait+refill,
context cancel, account isolation, bucket reset on RPM change.
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