docs: work on README

tests/AnthropicClient.Tests/Fixtures/ConfigurationFixture.cs

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+namespace AnthropicClient.Tests.Fixtures;
+
+public class ConfigurationFixture
+{
+  public string AnthropicApiKey { get; }
+
+  public ConfigurationFixture()
+  {
+    var configuration = new ConfigurationBuilder()
+      .AddJsonFile("appsettings.Test.json")
+      .AddEnvironmentVariables()
+      .Build();
+
+    var anthropicApiKey = configuration["AnthropicApiKey"];
+
+    if (string.IsNullOrEmpty(anthropicApiKey))
+    {
+      throw new InvalidOperationException("AnthropicApiKey is required");
+    }
+
+    AnthropicApiKey = anthropicApiKey;
+  }
+}