[MM-61463] Fix errcheck issues in post_helpers_test.go (#30609)

* [MM-61463] Fix errcheck issues in post_helpers_test.go

- Added proper error handling for System.Save calls
- Removed post_helpers_test.go from errcheck ignore list in .golangci.yml

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Add back         channels/app/plugin_test.go|

* Fix server/.golangci.yml

---------

Co-authored-by: Claude <noreply@anthropic.com>

server/.golangci.yml

@@ -102,7 +102,6 @@ issues:
         channels/app/platform/session.go|\
         channels/app/platform/status.go|\
         channels/app/platform/web_hub_test.go|\
-        channels/app/post_helpers_test.go|\
         channels/app/post_test.go|\
         channels/app/slack.go|\
         channels/app/slashcommands/auto_environment.go|\