[AZ-374] Refactor C21: named GoogleMapsTiles HttpClient in DI

- Register IHttpClientFactory named client "GoogleMapsTiles" inside
  AddTileDownloader() with User-Agent and 100s timeout (preserves
  HttpClient's implicit default).
- Resolve the same named client from all three CreateClient() call
  sites in GoogleMapsDownloaderV2 (session token, single-tile,
  batch-tile retry lambda) and drop the duplicated per-call
  UserAgent.ParseAdd setup.
- Expose USER_AGENT, the client name, and the timeout as internal
  consts on GoogleMapsDownloaderV2 so the extension and the
  downloader share one source of truth.
- Add AC test that builds the DI container, resolves the named
  client, and asserts both the User-Agent header and the timeout.
- Archive AZ-374 task file: todo/ -> done/.

175 unit + 5 smoke pass.

Co-authored-by: Cursor <cursoragent@cursor.com>

SatelliteProvider.Tests/TileServiceTests.cs

@@ -1,5 +1,6 @@
 using FluentAssertions;
 using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using Moq;
@@ -464,6 +465,30 @@ public class TileServiceTests
     }
 }
 
+public class TileDownloaderRegistrationTests
+{
+    [Fact]
+    public void AddTileDownloader_RegistersNamedGoogleMapsTilesHttpClient_AZ374_AC1()
+    {
+        // Arrange
+        var services = new ServiceCollection();
+
+        // Act
+        services.AddTileDownloader();
+        using var provider = services.BuildServiceProvider();
+        var factory = provider.GetRequiredService<IHttpClientFactory>();
+        using var client = factory.CreateClient("GoogleMapsTiles");
+
+        // Assert
+        client.DefaultRequestHeaders.UserAgent.Should().NotBeEmpty(
+            "AZ-374 AC-1: the named GoogleMapsTiles client must carry the User-Agent set once at registration time");
+        client.DefaultRequestHeaders.UserAgent.ToString()
+            .Should().Contain("Mozilla/5.0", "preserves the existing outbound User-Agent for Google Maps");
+        client.Timeout.Should().Be(TimeSpan.FromSeconds(100),
+            "preserves the HttpClient default (100s) until C18 wires this to MapConfig");
+    }
+}
+
 public class GoogleMapsDownloaderZoomValidationTests
 {
     private static GoogleMapsDownloaderV2 BuildDownloader()