Add TestWeixinSendImageFileParameterName test class with two tests:
- test_send_image_file_uses_image_path_parameter: verifies the correct
parameter name (image_path) is used when gateway calls send_image_file
- test_send_image_file_works_without_optional_params: ensures minimal
params work correctly
This prevents the interface from drifting again as noted by Copilot.