"Cooperative" is the word usually used here to describe the relationship between the campers of Occupy Philadelphia and the city, a birthplace of the constitutional right to free speech and assembly. The arguments and arrests that have occurred at protests in New York and other cities have been largely absent. Mayor Michael A. Nutter even visited the encampment on its first night and pledged to work with the movement when possible. But the limits of that cooperation are about to be tested. Following the example of other cities that have taken steps to evict the Occupy camps, Mr. Nutter, citing health and safety concerns and an imminent construction project, said the protesters must pack up and leave the steps of City Hall by Sunday evening.