postit: reverse geocode rdv map selections

src/PostIt/PostIt.Tests/NominatimReverseGeocodingServiceTests.cs

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+using System.Net;
+using System.Text;
+using PostIt.Services;
+
+namespace PostIt.Tests;
+
+public class NominatimReverseGeocodingServiceTests
+{
+    [Fact]
+    public async Task TryResolveAddressAsync_formats_compact_street_address_from_nominatim_payload()
+    {
+        var handler = new StubHandler("""
+            {
+              "display_name": "6, Place de l'Hôtel-de-Ville - Esplanade de la Libération, Paris, 75004, France",
+              "address": {
+                "house_number": "6",
+                "road": "Place de l'Hôtel-de-Ville - Esplanade de la Libération",
+                "postcode": "75004",
+                "city": "Paris"
+              }
+            }
+            """);
+
+        var service = new NominatimReverseGeocodingService(new HttpClient(handler)
+        {
+            BaseAddress = new Uri("https://nominatim.openstreetmap.org/")
+        });
+
+        var result = await service.TryResolveAddressAsync(48.8566, 2.3522);
+
+        Assert.Equal("6, Place de l'Hôtel-de-Ville - Esplanade de la Libération, 75004, Paris", result);
+        Assert.NotNull(handler.LastRequest);
+        Assert.Contains("reverse?format=jsonv2", handler.LastRequest!.RequestUri!.ToString(), StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public async Task TryResolveAddressAsync_returns_null_on_unsuccessful_response()
+    {
+        var handler = new StubHandler("{}", HttpStatusCode.TooManyRequests);
+        var service = new NominatimReverseGeocodingService(new HttpClient(handler)
+        {
+            BaseAddress = new Uri("https://nominatim.openstreetmap.org/")
+        });
+
+        var result = await service.TryResolveAddressAsync(48.8566, 2.3522);
+
+        Assert.Null(result);
+    }
+
+    private sealed class StubHandler : HttpMessageHandler
+    {
+        private readonly string _payload;
+        private readonly HttpStatusCode _statusCode;
+
+        public HttpRequestMessage? LastRequest { get; private set; }
+
+        public StubHandler(string payload, HttpStatusCode statusCode = HttpStatusCode.OK)
+        {
+            _payload = payload;
+            _statusCode = statusCode;
+        }
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            LastRequest = request;
+            return Task.FromResult(new HttpResponseMessage(_statusCode)
+            {
+                Content = new StringContent(_payload, Encoding.UTF8, "application/json")
+            });
+        }
+    }
+}