Clean up namespaces (#410)

No meaningful code changes, just lots of namespace shuffling to have fewer namespaces and to put things in better namespaces.

src/ModelContextProtocol/Protocol/PromptMessage.cs

@@ -0,0 +1,52 @@
+using Microsoft.Extensions.AI;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Represents a message within the Model Context Protocol (MCP) system, used for communication between clients and AI models.
+/// </summary>
+/// <remarks>
+/// <para>
+/// A <see cref="PromptMessage"/> encapsulates content sent to or received from AI models in the Model Context Protocol.
+/// Each message has a specific role (<see cref="Role.User"/> or <see cref="Role.Assistant"/>) and contains content which can be
+/// text, images, audio, or embedded resources.
+/// </para>
+/// <para>
+/// This class is similar to <see cref="SamplingMessage"/>, but with enhanced support for embedding resources from the MCP server.
+/// It serves as a core data structure in the MCP message exchange flow, particularly in prompt formation and model responses.
+/// </para>
+/// <para>
+/// <see cref="PromptMessage"/> objects are typically used in collections within <see cref="GetPromptResult"/> 
+/// to represent complete conversations or prompt sequences. They can be converted to and from <see cref="ChatMessage"/>
+/// objects using the extension methods <see cref="AIContentExtensions.ToChatMessage(PromptMessage)"/> and
+/// <see cref="AIContentExtensions.ToPromptMessages(ChatMessage)"/>.
+/// </para>
+/// <para>
+/// See the <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">schema</see> for details.
+/// </para>
+/// </remarks>
+public class PromptMessage
+{
+    /// <summary>
+    /// Gets or sets the content of the message, which can be text, image, audio, or an embedded resource.
+    /// </summary>
+    /// <remarks>
+    /// The <see cref="Content"/> object contains all the message payload, whether it's simple text,
+    /// base64-encoded binary data (for images/audio), or a reference to an embedded resource.
+    /// The <see cref="Content.Type"/> property indicates the specific content type.
+    /// </remarks>
+    [JsonPropertyName("content")]
+    public Content Content { get; set; } = new();
+
+    /// <summary>
+    /// Gets or sets the role of the message sender, specifying whether it's from a "user" or an "assistant".
+    /// </summary>
+    /// <remarks>
+    /// In the Model Context Protocol, each message must have a clear role assignment to maintain
+    /// the conversation flow. User messages represent queries or inputs from users, while assistant
+    /// messages represent responses generated by AI models.
+    /// </remarks>
+    [JsonPropertyName("role")]
+    public Role Role { get; set; } = Role.User;
+}