Send the play queue ID to the server

The server keeps a play queue per client, identified by the X-Queue-Id
header. The client sends the ID it has for the selected server with every
request and keeps the one in each response, which is new after a reset or a
sync. The IDs are saved per server in the [queues] section of settings.cfg,
so the game goes on with the same queue after a restart.

HttpTransport now passes the response headers on to the client.

tests/fakes/fake_http_transport.gd

@@ -12,6 +12,9 @@ extends HttpTransport
 ## Like the real server it hands out tokens on [code]POST /token[/code] and answers 401
 ## to requests without a valid one. Token requests are counted in
 ## [member token_requests], not in [member requested_paths].
+##
+## Set [member queue_id] to answer with an [code]X-Queue-Id[/code] header, like the
+## server's music endpoints.
 
 ## Paths that need no token, like on the real server.
 const OPEN_PATHS: Array[String] = ["/token", "/health"]
@@ -26,11 +29,14 @@ var requested_headers: Array[PackedStringArray] = []
 var token_requests: int = 0
 ## The JSON body of the last token request.
 var last_token_request: Dictionary = {}
+## Sent in an X-Queue-Id header with every answer that is not about a token, unless empty.
+var queue_id: String = ""
 
 var _responses: Dictionary = {}
 var _successes_left: Dictionary = {}
 var _held_paths: Dictionary = {}
 var _previous_transport: HttpTransport
+var _previous_queue_ids: Dictionary[String, String]
 var _valid_tokens: Array[String] = []
 
 
@@ -43,6 +49,8 @@ func install() -> FakeHttpTransport:
 	_previous_transport = MusicServerClient.transport
 	MusicServerClient.transport = self
 	MusicServerClient.clear_token()
+	_previous_queue_ids = Settings.queue_ids.duplicate()
+	Settings.queue_ids.clear()
 	return self
 
 
@@ -50,6 +58,7 @@ func install() -> FakeHttpTransport:
 func uninstall() -> void:
 	MusicServerClient.transport = _previous_transport
 	MusicServerClient.clear_token()
+	Settings.queue_ids = _previous_queue_ids
 
 
 ## Answers requests to [param path] with [param data] encoded as JSON.
@@ -112,7 +121,8 @@ func request(
 	requested_methods.append(method)
 	requested_headers.append(headers)
 	if !OPEN_PATHS.has(path.get_slice("?", 0)) && !_has_valid_token(headers):
-		on_complete.call(401, JSON.stringify({"error": "Unauthorized"}).to_utf8_buffer())
+		on_complete.call(401, JSON.stringify({"error": "Unauthorized"}).to_utf8_buffer(),
+				PackedStringArray())
 		return
 	if _held_paths.has(path):
 		_held_paths[path].append(_answer.bind(path, on_complete))
@@ -121,15 +131,18 @@ func request(
 
 
 func _answer(path: String, on_complete: Callable) -> void:
+	var headers: PackedStringArray = []
+	if !queue_id.is_empty() && path != "/token":
+		headers.append("X-Queue-Id: " + queue_id)
 	if _successes_left.has(path):
 		if _successes_left[path] == 0:
-			on_complete.call(503, PackedByteArray())
+			on_complete.call(503, PackedByteArray(), headers)
 			return
 		_successes_left[path] -= 1
 	if _responses.has(path):
-		on_complete.call(_responses[path].code, _responses[path].body)
+		on_complete.call(_responses[path].code, _responses[path].body, headers)
 	else:
-		on_complete.call(404, PackedByteArray())
+		on_complete.call(404, PackedByteArray(), headers)
 
 
 func _answer_token_request(body: String, on_complete: Callable) -> void:
@@ -147,7 +160,7 @@ func _answer_token_request(body: String, on_complete: Callable) -> void:
 	_valid_tokens.append(token)
 	on_complete.call(200, JSON.stringify({
 		"token": token, "expires_at": "2026-10-07T12:00:00Z", "client_type": "godot",
-	}).to_utf8_buffer())
+	}).to_utf8_buffer(), PackedStringArray())
 
 
 func _has_valid_token(headers: PackedStringArray) -> bool: