feat: add support for custom line edit icon margin

Added PM_LineEditIconMargin pixel metric handling to support custom
icon margins in line edit widgets. The implementation checks for a
"_d_dtk_lineeditIconMargin" property on the widget and returns the
custom margin value if valid. This allows applications to specify
precise icon spacing in line edit fields for better UI customization and
visual consistency.

Influence:
1. Test line edit widgets with custom icon margin property set
2. Verify default behavior when no custom margin is specified
3. Test with various valid margin values (positive integers)
4. Verify behavior with invalid margin values (negative numbers, invalid
types)
5. Check UI layout consistency with custom icon margins applied

feat: 添加支持自定义行编辑图标边距

添加了 PM_LineEditIconMargin 像素度量处理，以支持在行编辑部件中自定义图
标边距。该实现检查部件上的 "_d_dtk_lineeditIconMargin" 属性，如果有效则
返回自定义边距值。这允许应用程序为行编辑字段指定精确的图标间距，以实现更
好的 UI 定制和视觉一致性。

Influence:
1. 测试设置了自定义图标边距属性的行编辑部件
2. 验证未指定自定义边距时的默认行为
3. 使用各种有效边距值（正整数）进行测试
4. 验证无效边距值（负数、无效类型）时的行为
5. 检查应用自定义图标边距后的 UI 布局一致性

PMS: BUG-334965

Author: 18202781743

src/widgets/dstyle.cpp

@@ -2255,6 +2255,19 @@ int DStyle::pixelMetric(QStyle::PixelMetric m, const QStyleOption *opt, const QW
         return 16;
     case PM_MenuButtonIndicator:
         return DSizeModeHelper::element(8, QCommonStyle::pixelMetric(m, opt, widget));
+    // since Qt 6.3 or applied patch Add-setting-thc-ICON-size-attribute-in-lineedit-to-the-style-plugin.patch( uos or deepin).
+    case PM_LineEditIconMargin: {
+        if (widget) {
+            const QVariant &margin_value = widget->property("_d_dtk_lineeditIconMargin");
+            if (margin_value.isValid()) {
+                bool ok = false;
+                int margin = margin_value.toInt(&ok);
+                if (ok && margin >= 0) {
+                    return margin;
+                }
+            }
+        }
+    }
     case PM_FloatingButtonFrameMargin:
         return 3;
     default: