038 Federation Middle-Hop Retry Parity

Checklist

Reproduce and document the parity gap between local tunnel middle-hop runtime generation and federation-applied middle roles.
Update federation runtime apply logic so remote middle-hop services set handler retries when the next hop has multiple candidates.
Add regression coverage for federated middle-hop runtime generation or contract behavior, including multi-target fifo scenarios.
Verify release / cleanup paths remain correct when the federated middle service carries retry settings.
Run targeted backend tests for handler and federation contract coverage.

Local tunnel runtime generation now sets handler.retries for middle-hop services based on downstream candidate count in go-backend/internal/http/handler/mutations.go, which enables router-level re-selection after a failed primary node.
Federation runtime apply still creates remote middle-hop services without handler.retries in go-backend/internal/http/handler/federation.go, even though the remote chain hop itself uses the same selector failover settings (strategy, maxFails=1, failTimeout=10m).
Because go-gost/x/config/parsing/service/parse.go only enables router retries when cfg.Handler.Retries > 0, federated middle-hop services can still fail hard on the first offline primary target instead of switching to backup.
The gap creates inconsistent behavior: identical tunnel topologies can fail over correctly on local middle nodes but not on federated / remote middle nodes.

In go-backend/internal/http/handler/federation.go, compute retry budget for req.Role == "middle" from len(req.Targets) and set service["handler"]["retries"] to at least len(req.Targets) - 1 when there is more than one target.
Keep retry injection scoped to federated middle roles only; exit roles should continue to omit retries because they do not rebuild downstream chain selection.
Add regression coverage that proves federated middle runtime application preserves local parity, ideally by asserting the generated remote service config or by exercising a dual-panel contract path with multi-target middle nodes.
Recheck federation release behavior to ensure added retry fields do not affect idempotent cleanup, service deletion, or re-apply flows.