Server support (stdio) (#19)

* Server support (stdio)
* TestServer project
* Refactored McpClient class to extract shared endpoint logic (with McpServer) to a base class, McpJsonRpcEndpoint.
* Improved xmldoc comments for protocol types, mirroring specification schema comments.
* Update README.MD

src/mcpdotnet/Server/McpServerException.cs

@@ -0,0 +1,41 @@
+using McpDotNet.Client;
+
+namespace McpDotNet.Server;
+
+/// <summary>
+/// Represents errors that occur in the MCP server.
+/// </summary>
+public class McpServerException : Exception
+{
+    /// <summary>
+    /// Gets the error code if this exception was caused by a JSON-RPC error response.
+    /// </summary>
+    public int? ErrorCode { get; }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpServerException"/> class with a specified error message.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    public McpServerException(string message) : base(message)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpServerException"/> class with a specified error message and error code.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    /// <param name="errorCode">The error code associated with the JSON-RPC error response.</param>
+    public McpServerException(string message, int errorCode) : base(message)
+    {
+        ErrorCode = errorCode;
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="McpClientException"/> class with a specified error message and a reference to the inner exception that is the cause of this exception.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
+    /// <param name="innerException">The exception that is the cause of the current exception, or a null reference if no inner exception is specified.</param>
+    public McpServerException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}