Add dotnet activities for train interface.

enterprise/Activities/TrainActivities.cs

@@ -0,0 +1,40 @@
+using System.Net.Http.Json;
+using Temporalio.Activities;
+using TrainSearchWorker.Models;
+
+namespace TrainSearchWorker.Activities;
+
+public class TrainActivities
+{
+    private readonly HttpClient _client;
+
+    public TrainActivities(IHttpClientFactory clientFactory)
+    {
+        _client = clientFactory.CreateClient("TrainApi");
+    }
+
+    [Activity]
+    public async Task<List<Journey>> SearchTrains(SearchTrainsRequest request)
+    {
+        var response = await _client.GetAsync(
+            $"api/search?from={Uri.EscapeDataString(request.From)}" +
+            $"&to={Uri.EscapeDataString(request.To)}" +
+            $"&outbound_time={Uri.EscapeDataString(request.OutboundTime)}" +
+            $"&return_time={Uri.EscapeDataString(request.ReturnTime)}");
+
+        response.EnsureSuccessStatusCode();
+
+        return await response.Content.ReadFromJsonAsync<List<Journey>>() 
+            ?? throw new InvalidOperationException("Received null response from API");
+    }
+
+    [Activity]
+    public async Task<List<Journey>> BookTrains(BookTrainsRequest request)
+    {
+        var response = await _client.PostAsJsonAsync("api/book", request);
+        response.EnsureSuccessStatusCode();
+
+        return await response.Content.ReadFromJsonAsync<List<Journey>>()
+            ?? throw new InvalidOperationException("Received null response from API");
+    }
+}