test(server): 补齐长上下文计费字段契约

backend/internal/server/api_contract_test.go

@@ -593,6 +593,7 @@ func TestAPIContracts(t *testing.T) {
 						"total_cost": 0.5,
 						"actual_cost": 0.5,
 						"rate_multiplier": 1,
+						"long_context_billing_applied": false,
 						"billing_type": 0,
 							"stream": true,
 							"duration_ms": 100,