Add cross-platform import test project runnable on Linux

CADability.ImportTests targets net8.0 (no Windows suffix) and references
only CADability.csproj — no WinForms or OpenGL dependency — so all five
DXF import regression tests can run on Linux CI without the
Microsoft.WindowsDesktop.App runtime.

Also broadens the malformed-OBJECTS-section fallback catch from
DxfException to any Exception, since ACadSharp raises a
NullReferenceException (not DxfException) for a truncated OBJECTS section.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: claude

CADability/ImportDxf.cs

@@ -42,13 +42,13 @@ public Import(string fileName)
                 {
                     doc = new DxfReader(stream).Read();
                 }
-                catch (ACadSharp.Exceptions.DxfException)
+                catch (Exception)
                 {
                     // The OBJECTS section is the last DXF section and contains non-geometric
                     // data (dictionaries, layouts, xrecords). Geometry lives in HEADERS,
                     // TABLES, BLOCKS, and ENTITIES — all read before OBJECTS.
-                    // When OBJECTS is malformed we fall back to a read that stops there,
-                    // so that valid geometry is still imported.
+                    // When OBJECTS is malformed (DxfException, NullReferenceException, etc.)
+                    // we fall back to a read that stops before it, so geometry still imports.
                     doc = ReadWithoutObjectsSection(fileName);
                     if (doc == null) throw; // OBJECTS was not the problem — re-throw
                 }

tests/CADability.ImportTests/CADability.ImportTests.csproj

@@ -0,0 +1,32 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <IsPackable>false</IsPackable>
+    <Platforms>AnyCPU;x64</Platforms>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Remove="Files\**\*.*" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Content Include="Files\**\*.*">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.1.0" />
+    <PackageReference Include="MSTest.TestAdapter" Version="2.2.8" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.2.8" />
+    <PackageReference Include="coverlet.collector" Version="3.1.2" />
+    <PackageReference Include="JunitXml.TestLogger" Version="3.0.114" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\CADability\CADability.csproj" />
+  </ItemGroup>
+
+</Project>