feat(multiplayer): propagate match ID to an already-allocated pod via annotations

Investigated the Fleet-manifest wiring task flagged last commit and
found a deeper, previously-undesigned gap: Kubernetes env vars are
fixed at pod creation, but Agones allocates a match to an already-
running Ready pod well after it starts -- so there was no channel at
all for match-specific data (match ID) to reach that pod's processes.

Close it using the Agones GameServerAllocation API's documented
spec.metadata.annotations field, which Agones applies to the allocated
GameServer's own object_meta on success: server/agones.Client.Allocate
now requests cosmic-clash.io/match-id and cosmic-clash.io/allocation-id
annotations, and the supervisor reads them back from the same
/gameserver SDK call it already makes for the assigned port/address
(GameServer.ObjectMeta.Annotations), falling back to them for its own
control-plane registration only when MatchID isn't explicitly
configured -- an explicit value always wins, and a match ID resolvable
from neither source fails Start() closed before any HTTP call.

The exact object_meta vs objectMeta JSON key from a live Agones SDK
sidecar is not independently verified from this sandbox; documented
inline, and the fallback degrades safely (empty annotations map, same
as before this change) if it turns out to be wrong.

Covered by two new tests: the annotation actually flowing through to
the registration body, and fail-closed with neither config nor
annotation supplying a match ID (registerCalled stays false, not just
that Start() errors).

server/agones/allocation_test.go

@@ -28,6 +28,9 @@ func TestAllocateBuildsStrictGameServerAllocationAndEndpoint(t *testing.T) {
 		if body.APIVersion != "allocation.agones.dev/v1" || body.Kind != "GameServerAllocation" || len(body.Spec.Selectors) != 1 || body.Spec.Selectors[0].MatchLabels["cosmic-clash/region"] != "EU" {
 			t.Fatalf("body=%+v", body)
 		}
+		if body.Spec.Metadata.Annotations["cosmic-clash.io/match-id"] != "match-1" || body.Spec.Metadata.Annotations["cosmic-clash.io/allocation-id"] != "allocation-1" {
+			t.Fatalf("allocation did not request match/allocation ID annotations on the GameServer: %+v", body.Spec.Metadata.Annotations)
+		}
 		w.Header().Set("Content-Type", "application/json")
 		_, _ = w.Write([]byte(`{"status":{"state":"Allocated","gameServerName":"gs-a","address":"2001:db8::1","ports":[{"name":"default","port":7777}]}}`))
 	}))