fix(multiplayer): reset expiry on reauthentication

Game/scripts/control_plane_client.gd

@@ -87,6 +87,7 @@ func configure(url: String, token: String) -> bool:
 		return false
 	base_url = normalized
 	access_token = normalized_token
+	session_expires_at = ""
 	auth_expired = false
 	if _websocket != null:
 		connect_event_stream()

Game/tests/cases/test_control_plane_client.gd

@@ -53,6 +53,13 @@ func test_session_expiry_is_checked_at_the_boundary_and_fails_closed() -> void:
 	assert_true(not ControlPlaneClient.is_valid_rfc3339_timestamp("2026-08-31T12:00:00"), "timezone-less timestamp is rejected")
 
 
+func test_reconfiguration_discards_the_previous_session_expiry() -> void:
+	var client := ControlPlaneClient.new()
+	client.session_expires_at = "1970-01-01T00:00:01Z"
+	assert_true(client.configure("https://match.example", "new-session:opaque-token"), "new session configures successfully")
+	assert_eq(client.session_expires_at, "", "new credentials do not inherit the old expiry")
+
+
 func test_websocket_event_validation_requires_contract_specific_fields() -> void:
 	var envelope := {"event": "state_changed", "revision": 1, "resource_id": "ticket_123456789", "occurred_at": "2026-08-31T12:00:00Z", "state": "QUEUED"}
 	assert_true(ControlPlaneClient._valid_websocket_event(envelope), "valid state event is accepted")