Make memory stress size follow available RAM

audit/internal/platform/sat_test.go

@@ -276,6 +276,37 @@ func TestEnvIntFallback(t *testing.T) {
 	}
 }
 
+func TestMemoryStressSizeArgUsesAvailableMemory(t *testing.T) {
+	oldFreeMemBytes := satFreeMemBytes
+	satFreeMemBytes = func() int64 { return 96 * 1024 * 1024 * 1024 }
+	t.Cleanup(func() { satFreeMemBytes = oldFreeMemBytes })
+
+	if got := memoryStressSizeArg(); got != "65536M" {
+		t.Fatalf("sizeArg=%q want 65536M", got)
+	}
+}
+
+func TestMemoryStressSizeArgRespectsOverride(t *testing.T) {
+	oldFreeMemBytes := satFreeMemBytes
+	satFreeMemBytes = func() int64 { return 96 * 1024 * 1024 * 1024 }
+	t.Cleanup(func() { satFreeMemBytes = oldFreeMemBytes })
+	t.Setenv("BEE_VM_STRESS_SIZE_MB", "4096")
+
+	if got := memoryStressSizeArg(); got != "4096M" {
+		t.Fatalf("sizeArg=%q want 4096M", got)
+	}
+}
+
+func TestMemoryStressSizeArgFallsBackWhenFreeMemoryUnknown(t *testing.T) {
+	oldFreeMemBytes := satFreeMemBytes
+	satFreeMemBytes = func() int64 { return 0 }
+	t.Cleanup(func() { satFreeMemBytes = oldFreeMemBytes })
+
+	if got := memoryStressSizeArg(); got != "80%" {
+		t.Fatalf("sizeArg=%q want 80%%", got)
+	}
+}
+
 func TestClassifySATResult(t *testing.T) {
 	tests := []struct {
 		name   string