feat(grounds-gamemode): give matchmade servers a broker instead of a dialled port

[hbrombeer]

A matchmade server now waits for its matches on match.host.<gameServerName>.start rather than listening on a port the matchmaker dials. Agones names the pod after the GameServer, so the server derives that subject from its own HOSTNAME — which matters because these fleets run portPolicy: None and therefore have neither a Service nor a DNS name, leaving the matchmaker to read the PodIP off the GameServer status.

grpcPort and the matchhost container port stay for one release: a runtime image that has not been rebuilt has no NATS responder, and service-match falls back rather than letting the match form and then silently never start.

Only rendered when matchmaking is enabled, so an ordinary fleet is untouched. Verified with helm template.

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Landing order

This is one cutover across seven repos. The order is not cosmetic:

1. service-match feat/rest-api — serves REST alongside gRPC and pushes matches over NATS with a gRPC fallback. Safe on its own; nothing else may land before it is deployed.
2. charts feat/gamemode-nats — gives matchmade fleets NATS_URL. Inert until a runtime that reads it ships.
3. deploy feat/match-nats — gives service-match NATS_URL. Land together with the image bump to the release cut from (1).
4. grounds-minestom-runtime feat/matchhost-nats — removes the gRPC MatchHost server. Needs a release cut afterwards.
5. game-bedwars / duel feat/rest-client — REST clients; both also need to repin the runtime released in (4) to actually receive matches over NATS.
6. plugin-match feat/rest-client — REST client.

Only after every matchmade fleet answers on NATS: set GAMESERVER_GRPC_FALLBACK=false, then delete MatchGrpcService, quarkus-grpc and the MatchHost service block from the contract.