feat: add EmailClassifier with Claude API integration and response parsing

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

tests/SpamGuard.Tests/Services/EmailClassifierTests.cs

@@ -0,0 +1,100 @@
+// tests/SpamGuard.Tests/Services/EmailClassifierTests.cs
+namespace SpamGuard.Tests.Services;
+
+using System.Net;
+using System.Text.Json;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using SpamGuard.Configuration;
+using SpamGuard.Models;
+using SpamGuard.Services;
+
+public class EmailClassifierTests
+{
+    private static SpamGuardOptions DefaultOptions => new()
+    {
+        Claude = new ClaudeOptions
+        {
+            ApiKey = "test-key",
+            Model = "claude-sonnet-4-6",
+            MaxBodyLength = 2000
+        }
+    };
+
+    private static EmailSummary SampleEmail => new(
+        Uid: 1,
+        From: "spammer@sketchy.com",
+        Subject: "Buy now! Limited offer!",
+        BodySnippet: "Click here to claim your prize...",
+        Date: DateTimeOffset.UtcNow
+    );
+
+    [Fact]
+    public void BuildPrompt_ContainsSenderAndSubjectAndBody()
+    {
+        var classifier = new EmailClassifier(
+            Options.Create(DefaultOptions),
+            new NullLogger<EmailClassifier>(),
+            new HttpClient()
+        );
+
+        var prompt = classifier.BuildPrompt(SampleEmail);
+
+        Assert.Contains("spammer@sketchy.com", prompt);
+        Assert.Contains("Buy now! Limited offer!", prompt);
+        Assert.Contains("Click here to claim your prize...", prompt);
+    }
+
+    [Fact]
+    public void BuildPrompt_TruncatesLongBody()
+    {
+        var longBody = new string('x', 5000);
+        var email = SampleEmail with { BodySnippet = longBody };
+
+        var classifier = new EmailClassifier(
+            Options.Create(DefaultOptions),
+            new NullLogger<EmailClassifier>(),
+            new HttpClient()
+        );
+
+        var prompt = classifier.BuildPrompt(email);
+
+        // Body in prompt should be truncated to MaxBodyLength
+        Assert.DoesNotContain(longBody, prompt);
+    }
+
+    [Fact]
+    public void ParseResponse_ValidJson_ReturnsResult()
+    {
+        var json = """{"classification": "spam", "confidence": 0.95, "reason": "Unsolicited marketing"}""";
+
+        var result = EmailClassifier.ParseResponse(json);
+
+        Assert.NotNull(result);
+        Assert.True(result.IsSpam);
+        Assert.Equal(0.95, result.Confidence);
+        Assert.Equal("Unsolicited marketing", result.Reason);
+    }
+
+    [Fact]
+    public void ParseResponse_InvalidJson_ReturnsNull()
+    {
+        var result = EmailClassifier.ParseResponse("not json at all");
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void ParseResponse_JsonWithMarkdownFencing_ReturnsResult()
+    {
+        var json = """
+            ```json
+            {"classification": "legitimate", "confidence": 0.85, "reason": "Normal business email"}
+            ```
+            """;
+
+        var result = EmailClassifier.ParseResponse(json);
+
+        Assert.NotNull(result);
+        Assert.False(result.IsSpam);
+    }
+}