Use precise cache duration for "future beacon" error responses

[alienx5499]

Problem

When a client requests a round that is too far in the future, the relay returns a "Requested future beacon" (425 Too Early) response. The response was cached for the full chain period (info.Period), even when the next valid round would become available sooner. That over-caches the error and can serve a stale "future beacon" response longer than necessary.

Location: routes_handler.go – getBeacon() when round >= nextRound+1

Previous behavior: Cache duration was always -int64(info.Period) (full period in seconds).

Impact:

• Suboptimal caching: error responses cached for the whole period instead of until the next round
• Clients may see the "future beacon" error longer than needed
• A TODO in code noted this improvement

Solution

Compute the exact time until the next round and use that as the cache duration. Round N is at GenesisTime + (N-1)*Period; seconds from now until that time is used for the negative nextTime (cache duration) when returning the future-beacon error.

• Added SecondsUntilRound(round uint64) int64 on JsonInfoV2 in grpc/grpc.go: returns seconds from now until the given round, or 0 if the round is in the past or round is 0.
• In getBeacon(), when round >= nextRound+1, use info.SecondsUntilRound(nextRound) instead of info.Period for the cache duration.
• Removed the TODO about using a more precise nextTime.

Changes

grpc/grpc.go

• New method SecondsUntilRound(round uint64) int64 on JsonInfoV2: computes round time as GenesisTime + (round-1)*Period, returns max(0, roundTime - now).

routes_handler.go

• In the future-beacon branch: set preciseCacheSeconds := info.SecondsUntilRound(nextRound) and return -preciseCacheSeconds instead of -int64(info.Period).
• Removed the TODO comment.

grpc/grpc_test.go

• New test TestSecondsUntilRound: next round in 5s, round 0 returns 0, round already passed returns 0, round in 30s.

Testing

• go test ./grpc/... -v -run TestSecondsUntilRound
• go test ./grpc/... -v -run TestNextBeaconTime (unchanged)
• go build ./...

Notes

• Cache duration is now at most the time until the next round; it can be less than the full period when the next round is soon.
• No change to success paths or to when the "future beacon" error is returned; only the cache duration for that error is improved.