[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/channels/api4/job_test.go

@@ -280,9 +280,9 @@ func TestDownloadJob(t *testing.T) {
 		require.NoError(t, delErr, "Failed to delete job %s", job.Id)
 	}()
 
-	filePath := "./data/export/" + job.Id + "/testdat.txt"
-	mkdirAllErr := os.MkdirAll(filepath.Dir(filePath), 0770)
-	require.NoError(t, mkdirAllErr)
+	filePath := filepath.Join(*th.App.Config().FileSettings.Directory, "export", job.Id+"/testdat.txt")
+	err = os.MkdirAll(filepath.Dir(filePath), 0770)
+	require.NoError(t, err)
 
 	_, createErr := os.Create(filePath)
 	require.NoError(t, createErr)
@@ -314,9 +314,9 @@ func TestDownloadJob(t *testing.T) {
 
 	// Now we stub the results of the job into the same directory and try to download it again
 	// This time we should successfully retrieve the results without any error
-	filePath = "./data/export/" + job.Id + ".zip"
-	mkdirAllErr = os.MkdirAll(filepath.Dir(filePath), 0770)
-	require.NoError(t, mkdirAllErr)
+	filePath = filepath.Join(*th.App.Config().FileSettings.Directory, "export", job.Id+".zip")
+	err = os.MkdirAll(filepath.Dir(filePath), 0770)
+	require.NoError(t, err)
 
 	_, createErr = os.Create(filePath)
 	require.NoError(t, createErr)