LSP: fix messageRequest to not return nested title (#13674)

* LSP: fix window/showMessageRequest to not return nested title

* Add window/showMessageRequest handler to docs

runtime/doc/lsp.txt

@@ -176,6 +176,7 @@ specification. These LSP requests/notifications are defined by default:
     textDocument/typeDefinition*
     window/logMessage
     window/showMessage
+    window/showMessageRequest
     workspace/applyEdit
     workspace/symbol
 

runtime/lua/vim/lsp/handlers.lua

@@ -93,10 +93,7 @@ M['window/showMessageRequest'] = function(_, _, params)
   if choice < 1 or choice > #actions then
       return vim.NIL
   else
-    local action_chosen = actions[choice]
-    return {
-      title = action_chosen;
-    }
+    return actions[choice]
   end
 end