[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>

_docs/02_tasks/done/AZ-374_refactor_typed_httpclient_googlemaps.md

@@ -0,0 +1,62 @@
+# Refactor: register typed HttpClient for Google Maps in DI
+
+**Task**: AZ-374_refactor_typed_httpclient_googlemaps
+**Name**: Named HttpClient for GoogleMapsDownloaderV2
+**Description**: Register a named `HttpClient` ("GoogleMapsTiles") with default headers + timeout, and have `GoogleMapsDownloaderV2` resolve it via `IHttpClientFactory.CreateClient("GoogleMapsTiles")` everywhere.
+**Complexity**: 2 points
+**Dependencies**: None
+**Component**: Api + Services.TileDownloader
+**Tracker**: AZ-374
+**Epic**: AZ-350
+
+## Problem
+
+`SatelliteProvider.Services.TileDownloader/GoogleMapsDownloaderV2.cs:51, 107, 369` calls `_httpClientFactory.CreateClient()` in three different code paths (session-token fetch, single-tile download, batch-tile download retry lambda) and sets the `User-Agent` header per call. The factory pools `HttpMessageHandler`s correctly, but the per-call header setup is duplicated and error-prone.
+
+## Outcome
+
+- Single named-client registration in `Program.cs`.
+- All three downloader paths resolve `CreateClient("GoogleMapsTiles")`.
+- Same outbound HTTP behavior.
+- 37 unit + 5 smoke tests stay green.
+
+## Scope
+
+### Included
+- Add `services.AddHttpClient("GoogleMapsTiles", c => { c.DefaultRequestHeaders.UserAgent.ParseAdd(USER_AGENT); c.Timeout = TimeSpan.FromSeconds(<existing default>); });` in `Program.cs`.
+- Replace three `CreateClient()` calls with `CreateClient("GoogleMapsTiles")`.
+- Remove the per-call `UserAgent` setup.
+
+### Excluded
+- Migrating to a typed `HttpClient` subclass (deferred).
+- Adding Polly or retry policy at the factory level (existing manual retry stays).
+
+## Acceptance Criteria
+
+**AC-1: Single registration**
+Given the post-refactor `Program.cs`
+When inspected
+Then a single `AddHttpClient("GoogleMapsTiles", ...)` registration exists.
+
+**AC-2: Downloader uses the named client**
+Given the post-refactor downloader
+When grepped for `CreateClient(`
+Then all matches use the `"GoogleMapsTiles"` name.
+
+**AC-3: Tests stay green**
+Given the post-refactor build
+When `scripts/run-tests.sh --smoke` runs
+Then all 37 unit + 5 smoke scenarios pass.
+
+## Constraints
+
+- Same outbound `User-Agent` header text.
+- Same timeout (use the existing implicit default until C18 wires this to config).
+
+## Risks & Mitigation
+
+**Risk 1: timeout default surprises a slow path**
+- *Risk*: setting an explicit timeout may cut off a slow Google Maps response that previously hung indefinitely.
+- *Mitigation*: pick a generous default (e.g., 60 s) matching observed worst case; tune via C18.
+
+Full change entry: `_docs/04_refactoring/03-code-quality-refactoring/list-of-changes.md` (C21).