feat: add TrustedSenderStore with case-insensitive lookup and persistence

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

tests/SpamGuard.Tests/State/TrustedSenderStoreTests.cs

@@ -0,0 +1,79 @@
+// tests/SpamGuard.Tests/State/TrustedSenderStoreTests.cs
+namespace SpamGuard.Tests.State;
+
+using SpamGuard.State;
+
+public class TrustedSenderStoreTests : IDisposable
+{
+    private readonly string _tempDir;
+    private readonly TrustedSenderStore _store;
+
+    public TrustedSenderStoreTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"spamguard-test-{Guid.NewGuid()}");
+        Directory.CreateDirectory(_tempDir);
+        _store = new TrustedSenderStore(_tempDir);
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempDir))
+            Directory.Delete(_tempDir, true);
+    }
+
+    [Fact]
+    public void IsTrusted_ReturnsFalse_WhenEmpty()
+    {
+        Assert.False(_store.IsTrusted("someone@example.com"));
+    }
+
+    [Fact]
+    public void IsTrusted_ReturnsTrue_AfterAdd()
+    {
+        _store.Add("someone@example.com");
+        Assert.True(_store.IsTrusted("someone@example.com"));
+    }
+
+    [Fact]
+    public void IsTrusted_IsCaseInsensitive()
+    {
+        _store.Add("Someone@Example.COM");
+        Assert.True(_store.IsTrusted("someone@example.com"));
+    }
+
+    [Fact]
+    public void IsTrusted_TrimsWhitespace()
+    {
+        _store.Add("  someone@example.com  ");
+        Assert.True(_store.IsTrusted("someone@example.com"));
+    }
+
+    [Fact]
+    public void AddRange_AddsMultiple()
+    {
+        _store.AddRange(["a@b.com", "c@d.com", "e@f.com"]);
+        Assert.True(_store.IsTrusted("a@b.com"));
+        Assert.True(_store.IsTrusted("c@d.com"));
+        Assert.True(_store.IsTrusted("e@f.com"));
+    }
+
+    [Fact]
+    public void Save_And_Load_RoundTrips()
+    {
+        _store.Add("a@b.com");
+        _store.Add("c@d.com");
+        _store.Save();
+
+        var loaded = new TrustedSenderStore(_tempDir);
+        Assert.True(loaded.IsTrusted("a@b.com"));
+        Assert.True(loaded.IsTrusted("c@d.com"));
+        Assert.False(loaded.IsTrusted("x@y.com"));
+    }
+
+    [Fact]
+    public void Count_ReturnsCorrectValue()
+    {
+        _store.AddRange(["a@b.com", "c@d.com"]);
+        Assert.Equal(2, _store.Count);
+    }
+}