Improve testability, packaging, and docs

tests/LibNftables.Tests/NativeTestSupport.cs

@@ -0,0 +1,64 @@
+using System.Globalization;
+
+namespace LibNftables.Tests;
+
+internal static class NativeTestSupport
+{
+    internal static bool HasCapNetAdmin()
+    {
+        const int capNetAdminBit = 12;
+        const ulong mask = 1UL << capNetAdminBit;
+
+        try
+        {
+            foreach (var line in File.ReadLines("/proc/self/status"))
+            {
+                if (!line.StartsWith("CapEff:", StringComparison.Ordinal))
+                {
+                    continue;
+                }
+
+                var hex = line["CapEff:".Length..].Trim();
+                if (ulong.TryParse(hex, NumberStyles.AllowHexSpecifier, CultureInfo.InvariantCulture, out var value))
+                {
+                    return (value & mask) != 0;
+                }
+            }
+        }
+        catch
+        {
+            // If capability probing fails, keep tests conservative.
+        }
+
+        return false;
+    }
+
+    internal static bool TryCreateContext(out NftContext context)
+    {
+        try
+        {
+            context = new NftContext();
+            return true;
+        }
+        catch (DllNotFoundException)
+        {
+            context = null!;
+            return false;
+        }
+        catch (TypeInitializationException ex) when (ex.InnerException is DllNotFoundException)
+        {
+            context = null!;
+            return false;
+        }
+        catch (BadImageFormatException)
+        {
+            context = null!;
+            return false;
+        }
+        catch (EntryPointNotFoundException)
+        {
+            context = null!;
+            return false;
+        }
+    }
+}