MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/channels/app/integration_action_test.go

@@ -530,7 +530,7 @@ func TestPostActionProps(t *testing.T) {
 	require.Nil(t, err)
 	assert.True(t, len(clientTriggerId) == 26)
 
-	newPost, nErr := th.App.Srv().Store().Post().GetSingle(post.Id, false)
+	newPost, nErr := th.App.Srv().Store().Post().GetSingle(th.Context, post.Id, false)
 	require.NoError(t, nErr)
 
 	assert.True(t, newPost.IsPinned)