fix(gateway): put the opened game on the wire even when the enqueue is not yet matched

The real cause of 'Start game does not enter the game': encodeMatch gated the MatchResult game on matched (matched := m.Matched && m.Game != nil), so an open game awaiting an opponent (matched=false, game set) lost its game on the wire and the client had nothing to navigate into. Encode the game whenever m.Game is present; the backend's matched flag is authoritative. Regression test added (matched=false + game reaches the wire). The earlier codec fix guarded the same drop on the decode side.

gateway/internal/transcode/encode.go

@@ -166,14 +166,17 @@ func toWireState(s backendclient.StateResp) wire.StateView {
 // encodeMatch builds a MatchResult payload.
 func encodeMatch(m backendclient.MatchResp) []byte {
 	b := flatbuffers.NewBuilder(512)
-	matched := m.Matched && m.Game != nil
+	// Enqueue always lands the caller in a game; an open game awaiting an opponent reports
+	// matched=false but still carries it, so encode the game whenever it is present (else the
+	// client never receives it and cannot navigate in).
+	hasGame := m.Game != nil
 	var game flatbuffers.UOffsetT
-	if matched {
+	if hasGame {
 		game = buildGameView(b, *m.Game)
 	}
 	fb.MatchResultStart(b)
-	fb.MatchResultAddMatched(b, matched)
-	if matched {
+	fb.MatchResultAddMatched(b, m.Matched)
+	if hasGame {
 		fb.MatchResultAddGame(b, game)
 	}
 	b.Finish(fb.MatchResultEnd(b))