[MM-61099] Fix errcheck issues in server/channels/app/brand.go (#30679)

* [MM-28779] Fix errcheck issues in server/channels/app/brand.go

Remove brand.go from the errcheck exclusion list in .golangci.yml and fixed the error by properly handling the return value from a.MoveFile().

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

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

* [MM-28779] Add test to verify brand image backup functionality

Add a new test that verifies backup of the original brand image happens when a new one is uploaded. This helps to ensure the fix for errcheck issues is working as expected.

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

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

* use seperate temporary filestore for each test

* Use FileSettings.Directory instead of finding the dir programatically

* Fix another test

* Fix defer

* Update server/channels/api4/job_test.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix bad bot commit

* Cleanup logs message

* cleanup file path

* Fix error variable names

* WIP:cleanup panic ussage

* Revert "WIP:cleanup panic ussage"

This reverts commit c3284e4427a41c818acc161926cd2535dee9a6b9.

* cleanup error checks

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/.golangci.yml

@@ -89,7 +89,6 @@ issues:
         channels/api4/team_local.go|\
         channels/api4/websocket_test.go|\
         channels/app/bot_test.go|\
-        channels/app/brand.go|\
         channels/app/file_test.go|\
         channels/app/helper_test.go|\
         channels/app/permissions_test.go|\