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/api4/post_test.go

@@ -1645,7 +1645,7 @@ func TestPinPost(t *testing.T) {
 	_, err := client.PinPost(context.Background(), post.Id)
 	require.NoError(t, err)
 
-	rpost, appErr := th.App.GetSinglePost(post.Id, false)
+	rpost, appErr := th.App.GetSinglePost(th.Context, post.Id, false)
 	require.Nil(t, appErr)
 	require.True(t, rpost.IsPinned, "failed to pin post")
 
@@ -1675,7 +1675,7 @@ func TestUnpinPost(t *testing.T) {
 	_, err := client.UnpinPost(context.Background(), pinnedPost.Id)
 	require.NoError(t, err)
 
-	rpost, appErr := th.App.GetSinglePost(pinnedPost.Id, false)
+	rpost, appErr := th.App.GetSinglePost(th.Context, pinnedPost.Id, false)
 	require.Nil(t, appErr)
 	require.False(t, rpost.IsPinned)