[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.Services.TileDownloader/TileDownloaderServiceCollectionExtensions.cs

@@ -8,6 +8,11 @@ public static class TileDownloaderServiceCollectionExtensions
     public static IServiceCollection AddTileDownloader(this IServiceCollection services)
     {
         services.AddMemoryCache();
+        services.AddHttpClient(GoogleMapsDownloaderV2.GoogleMapsTilesClientName, c =>
+        {
+            c.DefaultRequestHeaders.UserAgent.ParseAdd(GoogleMapsDownloaderV2.UserAgent);
+            c.Timeout = TimeSpan.FromSeconds(GoogleMapsDownloaderV2.DefaultHttpClientTimeoutSeconds);
+        });
         services.AddSingleton<ISatelliteDownloader, GoogleMapsDownloaderV2>();
         services.AddSingleton<ITileService, TileService>();
         return services;