Fix concurrency for CPU mode

Prior refactoring passes accidentally removed the logic to bypass VRAM
checks for CPU loads.  This adds that back, along with test coverage.

This also fixes loaded map access in the unit test to be behind the mutex which was
likely the cause of various flakes in the tests.

server/sched.go

@@ -149,6 +149,14 @@ func (s *Scheduler) processPending(ctx context.Context) {
 						break
 					}
 
+					// If we're CPU only mode, just limit by loadedMax above
+					// TODO handle system memory exhaustion
+					if (len(gpus) == 1 && gpus[0].Library == "cpu") || pending.opts.NumGPU == 0 {
+						slog.Debug("cpu mode with existing models, loading")
+						s.loadFn(pending, ggml, gpus)
+						break
+					}
+
 					// No models loaded. Load the model but prefer the best fit.
 					if loadedCount == 0 {
 						slog.Debug("loading first model", "model", pending.model.ModelPath)