fix: PA pattern calibration crash with first layer line width 0 (#14447)

CalibPressureAdvancePattern::line_width_first_layer() returned the raw
initial_layer_line_width, so a value of 0 (which means "use the default")
fed 0 into the Flow spacing math and threw FlowErrorNegativeSpacing,
crashing the whole app when slicing a PA pattern calibration.

Mirror the guard already present in the sibling line_width(): when the
configured width is non-positive, fall back to auto_extrusion_width.

Add a libslic3r regression test covering the width resolution.

Fixes #13188

tests/libslic3r/CMakeLists.txt

@@ -8,6 +8,7 @@ add_executable(${_TEST_NAME}_tests
     test_arachne_walls.cpp
     test_arrange.cpp
     test_bambu_networking.cpp
+    test_calib.cpp
     test_clipper_offset.cpp
     test_clipper_utils.cpp
     test_config.cpp

tests/libslic3r/test_calib.cpp

@@ -0,0 +1,40 @@
+#include <catch2/catch_all.hpp>
+
+#include "libslic3r/calib.hpp"
+#include "libslic3r/Model.hpp"
+#include "libslic3r/TriangleMesh.hpp"
+#include "libslic3r/PrintConfig.hpp"
+
+using namespace Slic3r;
+
+namespace {
+
+// The width-resolution getters are protected; expose them so the resolution can be asserted directly.
+struct PaPatternProbe : public CalibPressureAdvancePattern
+{
+    using CalibPressureAdvancePattern::CalibPressureAdvancePattern;
+    using CalibPressureAdvancePattern::line_width;
+    using CalibPressureAdvancePattern::line_width_first_layer;
+};
+
+} // namespace
+
+TEST_CASE("Zero calibration line width resolves to a positive default", "[Calib][Regression]")
+{
+    DynamicPrintConfig config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({
+        {"line_width", "0"},
+        {"initial_layer_line_width", "0"},
+    });
+
+    Model model;
+    model.add_object("cube", "", make_cube(20, 20, 20))->add_instance();
+
+    Calib_Params params;
+    params.mode = CalibMode::Calib_PA_Pattern;
+
+    PaPatternProbe pattern(params, config, /* is_bbl_machine */ true, *model.objects.front(), Vec3d(0, 0, 0));
+
+    REQUIRE(pattern.line_width() > 0.);
+    REQUIRE(pattern.line_width_first_layer() > 0.);
+}