docs: work on README

tests/AnthropicClient.Tests/Unit/Models/MessageTests.cs

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+namespace AnthropicClient.Tests.Unit.Models;
+
+public class MessageTests : SerializationTest
+{
+  private readonly string _testJson = @"{
+    ""role"": ""assistant"",
+    ""content"": [
+      { ""text"": ""text"", ""type"": ""text"" }
+    ]
+  }";
+
+  [Fact]
+  public void Constructor_WhenCalled_ItShouldInitializeProperties()
+  {
+    var role = "assistant";
+    var content = new List<Content> { new TextContent("text") };
+
+    var message = new Message(role, content);
+
+    message.Role.Should().Be(role);
+    message.Content.Should().BeSameAs(content);
+  }
+
+  [Fact]
+  public void Constructor_WhenCalledAndRoleIsNull_ItShouldThrowArgumentNullException()
+  {
+    var content = new List<Content> { new TextContent("text") };
+
+    var action = () => new Message(null!, content);
+
+    action.Should().Throw<ArgumentNullException>();
+  }
+
+  [Fact]
+  public void Constructor_WhenCalledAndContentIsNull_ItShouldThrowArgumentNullException()
+  {
+    var role = "assistant";
+
+    var action = () => new Message(role, null!);
+
+    action.Should().Throw<ArgumentNullException>();
+  }
+
+  [Fact]
+  public void Constructor_WhenCalledAndRoleIsInvalid_ItShouldThrowArgumentException()
+  {
+    var role = "invalid";
+    var content = new List<Content> { new TextContent("text") };
+
+    var action = () => new Message(role, content);
+
+    action.Should().Throw<ArgumentException>();
+  }
+
+  [Fact]
+  public void JsonSerialization_WhenSerialized_ItShouldReturnJsonString()
+  {
+    var role = "assistant";
+    var content = new List<Content> { new TextContent("text") };
+    var message = new Message(role, content);
+
+    var actual = Serialize(message);
+
+    JsonAssert.Equal(_testJson, actual);
+  }
+
+  [Fact]
+  public void JsonDeserialization_WhenDeserialized_ItShouldReturnMessage()
+  {
+    var message = Deserialize<Message>(_testJson);
+
+    message.Should().NotBeNull();
+    message!.Role.Should().Be("assistant");
+    message.Content.Should().HaveCount(1);
+    message.Content[0].Should().BeOfType<TextContent>();
+    message.Content[0].As<TextContent>().Text.Should().Be("text");
+  }
+}