The contract that keeps thousands of dockworkers on the job at West Coast ports from San Diego to Seattle is expiring, but don’t expect a disruption in the billions of dollars of trade that crosses the waterfront — at least not yet. Both the ILWU and the association that represents shipping lines and terminal operators at 29 ports have promised to keep negotiating past 5 p.m. PDT Tuesday, when the current six-year contract ends. Indeed, no resolution is expected for weeks, with tough issues including whether workers should shoulder more of the hefty cost of health care yet to be resolved.