sched: logging improvements (#10550)

This enhances our logging in the scheduler.  The initial "waiting for server" log
no longer claims an initial error state (now "not responding" which better reflects
the actual state).  Runners now have slog wiring to report more details about the
runner, including PID.

server/sched_test.go

@@ -792,3 +792,4 @@ func (s *mockLlm) Close() error {
 func (s *mockLlm) EstimatedVRAM() uint64                  { return s.estimatedVRAM }
 func (s *mockLlm) EstimatedTotal() uint64                 { return s.estimatedTotal }
 func (s *mockLlm) EstimatedVRAMByGPU(gpuid string) uint64 { return s.estimatedVRAMByGPU[gpuid] }
+func (s *mockLlm) Pid() int                               { return -1 }