Handle requests for capability, when capability not present

Will also lower log level from err to wrn

Scenario:
ex: Claude Desktop polls for `/resources/list` even though `resources` is missing from the `capabilities`

Author: johnkors

src/ModelContextProtocol/Shared/McpSession.cs

@@ -298,7 +298,17 @@ private void HandleMessageWithId(JsonRpcMessage message, JsonRpcMessageWithId me
         if (!_requestHandlers.TryGetValue(request.Method, out var handler))
         {
             LogNoHandlerFoundForRequest(EndpointName, request.Method);
-            throw new McpException($"Method '{request.Method}' is not available.", McpErrorCode.MethodNotFound);
+            await SendMessageAsync(new JsonRpcError
+            {
+                Id = request.Id,
+                JsonRpc = "2.0",
+                Error = new JsonRpcErrorDetail
+                {
+                    Message = $"{EndpointName} lacks capability to do {request.Method}",
+                    Code = 400
+                },
+            }, cancellationToken).ConfigureAwait(false);
+            return null;
         }
 
         LogRequestHandlerCalled(EndpointName, request.Method);
@@ -764,4 +774,4 @@ private static TimeSpan GetElapsed(long startingTimestamp) =>
 
     [LoggerMessage(Level = LogLevel.Trace, Message = "{EndpointName} sending message. Message: '{Message}'.")]
     private partial void LogSendingMessageSensitive(string endpointName, string message);
-}
+}