Update McpErrorCode XML docs and fix error code usage to align with MCP spec

src/ModelContextProtocol.Core/McpErrorCode.cs

@@ -39,10 +39,19 @@ public enum McpErrorCode
     InvalidRequest = -32600,
 
     /// <summary>
-    /// Indicates that the requested method does not exist or is not available on the server.
+    /// Indicates that the requested method does not exist or is not available.
     /// </summary>
     /// <remarks>
-    /// This error is returned when the method name specified in the request cannot be found.
+    /// <para>
+    /// In MCP, this error is returned when a request is made for a method that requires a capability
+    /// that has not been declared. This can occur in either direction:
+    /// </para>
+    /// <list type="bullet">
+    /// <item><description>A server returning this error when the client requests a capability it doesn't support
+    /// (for example, requesting completions when the <c>completions</c> capability was not advertised).</description></item>
+    /// <item><description>A client returning this error when the server requests a capability it doesn't support
+    /// (for example, requesting roots when the client did not declare the <c>roots</c> capability).</description></item>
+    /// </list>
     /// </remarks>
     MethodNotFound = -32601,
 
@@ -51,12 +60,16 @@ public enum McpErrorCode
     /// </summary>
     /// <remarks>
     /// <para>
-    /// This error is returned for protocol-level parameter issues, such as:
+    /// In MCP, this error is returned in various contexts when request parameters fail validation:
     /// </para>
     /// <list type="bullet">
-    /// <item><description>Malformed requests that fail to satisfy the request schema (for example, CallToolRequest)</description></item>
-    /// <item><description>Unknown or unrecognized primitive names (for example, tool, prompt, or resource names)</description></item>
-    /// <item><description>Missing required protocol-level parameters</description></item>
+    /// <item><description><b>Tools</b>: Unknown tool name or invalid tool arguments.</description></item>
+    /// <item><description><b>Prompts</b>: Unknown prompt name or missing required arguments.</description></item>
+    /// <item><description><b>Pagination</b>: Invalid or expired cursor values.</description></item>
+    /// <item><description><b>Logging</b>: Invalid log level.</description></item>
+    /// <item><description><b>Tasks</b>: Invalid or nonexistent task ID, invalid cursor, or attempting to cancel a task already in a terminal status.</description></item>
+    /// <item><description><b>Elicitation</b>: Server requests an elicitation mode not declared in client capabilities.</description></item>
+    /// <item><description><b>Sampling</b>: Missing tool result or tool results mixed with other content.</description></item>
     /// </list>
     /// <para>
     /// Note: Input validation errors within tool/prompt/resource arguments should be reported as execution errors