Improve testability, packaging, and docs

tests/LibNftables.Tests/NftErrorTranslatorTests.cs

@@ -0,0 +1,41 @@
+namespace LibNftables.Tests;
+
+public sealed class NftErrorTranslatorTests
+{
+    [Fact]
+    public void PermissionErrors_AreClassified()
+    {
+        NftException ex = NftErrorTranslator.FromOperationFailure("RunCommand", 1, "Operation not permitted");
+
+        Assert.IsType<NftPermissionException>(ex);
+        Assert.Equal(1, ex.NativeErrorCode);
+    }
+
+    [Fact]
+    public void ValidationErrors_AreClassified()
+    {
+        NftException ex = NftErrorTranslator.FromOperationFailure("RunCommand", 2, "syntax error, unexpected token");
+
+        Assert.IsType<NftValidationException>(ex);
+        Assert.Equal(2, ex.NativeErrorCode);
+    }
+
+    [Fact]
+    public void UnsupportedErrors_AreClassified()
+    {
+        NftException ex = NftErrorTranslator.FromOperationFailure("RunCommand", 3, "operation not supported");
+
+        Assert.IsType<NftUnsupportedException>(ex);
+        Assert.Equal(3, ex.NativeErrorCode);
+    }
+
+    [Fact]
+    public void UnknownErrors_FallBackToBaseException()
+    {
+        NftException ex = NftErrorTranslator.FromOperationFailure("RunCommand", 4, "unexpected native failure");
+
+        Assert.IsType<NftException>(ex);
+        Assert.Equal(4, ex.NativeErrorCode);
+        Assert.DoesNotContain("syntax error", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+}