feat(presence): count players by server across the network

[hbrombeer]

Why

Velocity can only count the players connected to the proxy that is asking. With
two proxies in front of one lobby, /agones reported "1 player" in a lobby
that actually held two — the second player was on the other proxy. service-player
holds the network-wide truth, so anything that states a number about the
network (/agones, a server selector) has to ask it instead of counting
locally.

This adds countPlayersByServer() through the presence chain so
ProxyService can answer for the whole network:

GrpcPlayerPresenceClient.countPlayersByServer()  (common, raw gRPC call)
  -> PlayerPresenceService.countPlayersByServer() (velocity, wrapper)
    -> PlayerSessionQueryImpl.countPlayersByServer() (velocity, implements plugin-proxy's PlayerSessionQuery)

Each layer mirrors the existing getPlayerSession / resolvePlayerName /
suggestNames lookups: same deadline handling, same fail-soft error handling.
On a gRPC failure the method returns null, never an empty
NetworkPlayerCounts — empty means "nobody is online anywhere", null means
"we could not ask the network", and callers must not substitute proxy-local
numbers for either.

Status

This is step 4 of 5 in the cross-proxy player-count rollout, and is a
draft because it depends on two releases that have not shipped yet:

• library-grpc-contracts-player 0.4.0 (adds the CountPlayersByServer RPC) — feat(player): count online players per backend server library-grpc-contracts#71
• plugin-proxy-api 0.2.0 (adds PlayerSessionQuery.countPlayersByServer() + NetworkPlayerCounts) — feat(proxy): expose network-wide player counts per backend server plugin-proxy#15

Both dependency versions are already bumped in this PR and resolve
correctly (verified locally against mavenLocal()), but GitHub Packages
does not have them published yet, so CI will not be able to resolve
dependencies until those two are released. This PR should stay in draft
until then.

Test plan

• ./gradlew build — BUILD SUCCESSFUL, all tests pass (verified locally with a temporary mavenLocal() repository, removed before commit)
• New test: a CountPlayersByServerReply with two servers maps to the right byServer map and carries total through
• New test: an unreachable/unavailable presence service yields null from PlayerSessionQueryImpl.countPlayersByServer(), not an empty NetworkPlayerCounts