Test that max_concurrent_processing defaults to at most 4

The default is the number of CPUs Go uses, so a container with no CPU
limit on a host with many CPUs processes one image per CPU at once. These
tests fail until the default in the config and in imageprocessor.New is
held to 4 (#242).

Model: opus-5-5

internal/imageprocessor/max_concurrent_processing_internal_test.go

@@ -132,6 +132,27 @@ func TestNewDefaultsMaxConcurrentProcessingToCPUs(t *testing.T) {
 	}
 }
 
+// TestNewDefaultMaxConcurrentProcessingIsAtMostFour checks that New, given
+// no MaxConcurrentProcessing, makes one slot per CPU Go uses, but never more
+// than 4.
+func TestNewDefaultMaxConcurrentProcessingIsAtMostFour(t *testing.T) {
+	t.Parallel()
+
+	want := min(runtime.GOMAXPROCS(0), 4)
+
+	for _, limit := range []int{0, -1} {
+		proc := New(Params{MaxConcurrentProcessing: limit})
+		got := cap(proc.processingSemaphore)
+		t.Logf("MaxConcurrentProcessing %d, GOMAXPROCS %d: %d slots",
+			limit, runtime.GOMAXPROCS(0), got)
+
+		if got != want {
+			t.Errorf("MaxConcurrentProcessing %d: %d slots, want %d",
+				limit, got, want)
+		}
+	}
+}
+
 // TestProcessNeverExceedsMaxConcurrentProcessing starts more Process calls
 // than MaxConcurrentProcessing allows and holds the first ones inside
 // Process until the test lets them go. No more than the limit may be