feat: Add Gmail Catchall Labeler service with configuration and functionality

- Introduced a new Docker Compose configuration for the Gmail Catchall Labeler service.
- Implemented GmailOptions class for managing configuration settings.
- Developed GmailApiClient to interact with the Gmail API, including methods for fetching messages and managing labels.
- Created models for Gmail messages, labels, and headers.
- Added IGmailClient interface to define the contract for Gmail client implementations.
- Implemented GmailOrganizer to handle the organization of emails based on recipient addresses.
- Developed LabelNameResolver for generating label names based on local parts of email addresses.
- Created RecipientExtractor to extract local parts from email headers.
- Implemented state management with OrganizerState and StateStore for persisting state information.
- Added Worker class to manage the background processing of email organization.
- Created unit tests for GmailOrganizer, LabelNameResolver, and RecipientExtractor to ensure functionality.
- Added necessary project files and configurations for building and running the application.

tests/GmailCatchallLabeler.Tests/RecipientExtractorTests.cs

@@ -0,0 +1,57 @@
+using GmailCatchallLabeler.Configuration;
+using GmailCatchallLabeler.Gmail;
+using GmailCatchallLabeler.Labels;
+
+namespace GmailCatchallLabeler.Tests;
+
+public sealed class RecipientExtractorTests
+{
+    private readonly RecipientExtractor extractor = new(new GmailOptions
+    {
+        CatchallDomain = "arnemoerman.be"
+    });
+
+    [Fact]
+    public void ExtractLocalParts_ReadsDeliveryAndRecipientHeaders()
+    {
+        GmailHeader[] headers =
+        [
+            new("Delivered-To", "tomscott@arnemoerman.be"),
+            new("To", "Example <every-foods@arnemoerman.be>, other@example.com"),
+            new("Subject", "Ignored")
+        ];
+
+        var result = extractor.ExtractLocalParts(headers);
+
+        Assert.Equal(2, result.Count);
+        Assert.Contains("tomscott", result);
+        Assert.Contains("every-foods", result);
+    }
+
+    [Fact]
+    public void ExtractLocalParts_IsCaseInsensitiveAndDeduplicates()
+    {
+        GmailHeader[] headers =
+        [
+            new("X-Original-To", "News@ArneMoerman.be"),
+            new("Cc", "news@arnemoerman.be")
+        ];
+
+        var result = extractor.ExtractLocalParts(headers);
+
+        Assert.Single(result);
+        Assert.Contains("News", result);
+    }
+
+    [Fact]
+    public void ExtractLocalParts_IgnoresOtherDomainsAndMalformedHeaders()
+    {
+        GmailHeader[] headers =
+        [
+            new("To", "other@example.com"),
+            new("Envelope-To", "not an address <")
+        ];
+
+        Assert.Empty(extractor.ExtractLocalParts(headers));
+    }
+}