Add tests for handling ambiguous timeout errors and deferred relay side effects in Self-serve entrance start logic

- Introduced `SelfserveLaneStartEntranceTimeoutHarness` class and supporting tests to validate ambiguous relay timeout handling during entrance operations.
- Added `defer_relay_side_effects` parameter to `selfserve_lane_command_arguments` for improved relay control during wash start.
- Enhanced lane start routine to support conditional relay side effects and timeout handling with detailed logging.

services/nginx/app/openapi.yaml

@@ -8900,6 +8900,13 @@ paths:
                 license_plate:
                   type: string
                   description: Required for START command
+                customer_number:
+                  type: integer
+                  description: Required for START and RESERVE commands. The authenticated customer's number is applied server-side when omitted by user clients.
+                defer_relay_side_effects:
+                  type: boolean
+                  default: false
+                  description: For START, open the entrance gate as part of the command but defer cleaner and machine relay activation to explicit relay endpoints.
       responses:
         '200':
           description: Command sent successfully