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

@@ -282,7 +282,17 @@ private async Task HandleRequest(JsonRpcRequest request, CancellationToken cance
         if (!_requestHandlers.TryGetValue(request.Method, out var handler))
         {
             _logger.NoHandlerFoundForRequest(EndpointName, request.Method);
-            throw new McpException("The method does not exist or is not available.", ErrorCodes.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;
         }
 
         _logger.RequestHandlerCalled(EndpointName, request.Method);
@@ -614,4 +624,4 @@ private static TimeSpan GetElapsed(long startingTimestamp) =>
 #else
         new((long)(s_timestampToTicks * (Stopwatch.GetTimestamp() - startingTimestamp)));
 #endif
-}
+}