#794 JumperClient singleton ignores queue_name parameter causing queue contamination

Root Cause: get_jumper_client() singleton caches queue_name from first call, ignores subsequent. When /retry endpoint called for workflow on highway_internal queue, singleton initialized with that queue. All subsequent submissions used contaminated singleton routing to highway_internal. Impact: Production workflows after 18:14 on 2025-01-02 went to highway_internal instead of highway_default. Fix: Per-queue singletons using dict[queue_name, JumperClient].