feat: implement generate command in console app

src/AltGen.Console/Generate/AltGenService.cs

@@ -0,0 +1,44 @@
+namespace AltGen.Console.Generate;
+
+sealed class AltGenService(HttpClient client) : IAltGenService
+{
+  static readonly JsonSerializerOptions JsonOptions = new()
+  {
+    PropertyNameCaseInsensitive = true
+  };
+  readonly HttpClient _client = client;
+
+  public async Task<string> GenerateAltTextAsync(GenerateAltTextRequest req)
+  {
+    var byteContent = new ByteArrayContent(req.Image);
+    byteContent.Headers.ContentType = new MediaTypeHeaderValue(req.ContentType);
+
+    var request = new HttpRequestMessage(HttpMethod.Post, $"{Constants.AltGenApiUri}/generate")
+    {
+      Content = new MultipartFormDataContent
+      {
+        { new StringContent(req.Provider), "provider" },
+        { new StringContent(req.ProviderKey), "providerKey" },
+        { new ByteArrayContent(req.Image), "file", req.FileName }
+      }
+    };
+
+    var response = await _client.SendAsync(request);
+    var content = await response.Content.ReadAsStringAsync();
+
+    if (response.IsSuccessStatusCode is false)
+    {
+      throw new AltTextException("Failed to generate alt text.");
+    }
+
+    var altTextResponse = JsonSerializer.Deserialize<AltTextResponse>(content, JsonOptions) ?? throw new AltTextException("Failed to deserialize response.");
+
+    return altTextResponse.AltText;
+  }
+}
+
+record AltTextResponse(string AltText);
+
+class AltTextException(string message) : Exception(message)
+{
+}