Ports the gateway interest-only mode from Go (gateway.go:100-150, 1500-1600):
- Add GatewayInterestTracker with Optimistic/Transitioning/InterestOnly modes
- In Optimistic mode, track no-interest set; switch to InterestOnly when set
exceeds threshold (default 1000, matching Go defaultGatewayMaxRUnsubThreshold)
- In InterestOnly mode, only forward subjects with tracked RS+ interest;
use SubjectMatch.MatchLiteral for wildcard pattern support
- Integrate tracker into GatewayConnection: A+/A- messages update tracker,
SendMessageAsync skips send when ShouldForward returns false
- Expose InterestTracker property on GatewayConnection for observability
- Add 13 unit tests covering all 8 specified behaviors plus edge cases
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