Add src files

tests/LibNftables.Tests/NftablesClientTests.cs

@@ -0,0 +1,66 @@
+namespace LibNftables.Tests;
+
+public sealed class NftablesClientTests
+{
+    [Fact]
+    public void Apply_WithBothTextAndFile_ThrowsValidationException()
+    {
+        var client = new NftablesClient();
+        var request = new NftApplyRequest
+        {
+            RulesetText = "flush ruleset",
+            RulesetFilePath = "/tmp/does-not-matter.nft",
+        };
+
+        Assert.Throws<NftValidationException>(() => client.Apply(request));
+    }
+
+    [Fact]
+    public void Validate_InvalidRuleset_ReturnsInvalidResult()
+    {
+        var client = new NftablesClient();
+        var request = NftApplyRequest.FromText("this is not valid nft syntax");
+
+        NftValidationResult result = client.Validate(request);
+
+        Assert.False(result.IsValid);
+        Assert.False(string.IsNullOrWhiteSpace(result.Diagnostics));
+    }
+
+    [Fact]
+    public async System.Threading.Tasks.Task ValidateAsync_InvalidRuleset_ReturnsInvalidResult()
+    {
+        var client = new NftablesClient();
+        var request = NftApplyRequest.FromText("this is not valid nft syntax");
+
+        NftValidationResult result = await client.ValidateAsync(request);
+
+        Assert.False(result.IsValid);
+        Assert.False(string.IsNullOrWhiteSpace(result.Diagnostics));
+    }
+
+    [Fact]
+    public void Apply_InvalidRuleset_ThrowsValidationException()
+    {
+        var client = new NftablesClient();
+        var request = NftApplyRequest.FromText("this is not valid nft syntax");
+
+        Assert.Throws<NftValidationException>(() => client.Apply(request));
+    }
+
+    [Fact]
+    public void Snapshot_WithInsufficientPrivileges_ThrowsPermissionOrReturnsRuleset()
+    {
+        var client = new NftablesClient();
+
+        try
+        {
+            NftSnapshot snapshot = client.Snapshot();
+            Assert.False(string.IsNullOrWhiteSpace(snapshot.RulesetText));
+        }
+        catch (NftPermissionException)
+        {
+            // Expected in unprivileged environments.
+        }
+    }
+}