MM-60480: Do not invalidate channel member cache on login (#28143)

On user login, we were aggressively clearing the channel member
cache when it wasn't necessary. There is no channel membership
info that is changing on a user login.

Additionally, we fix some more issues with pointer passing
to the cache. It's a known problem that our value passing
style isn't consistent when we fetch items from cache.
Sometimes we pass pointer-to-pointer, sometimes it's just a
pointer. This is also the reason for https://github.com/mattermost/mattermost/pull/27830.

We fix 2 such cases where we passed pointer-to-pointer
but didn't handle the special case inside the cache.
This time, we actually fix it in the app layer instead
of the cache layer and also add a test for good measure.

https://mattermost.atlassian.net/browse/MM-60480

```release-note
NONE
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/localcachelayer/channel_layer.go

@@ -291,7 +291,7 @@ func (s LocalCacheChannelStore) GetAllChannelMembersForUser(ctx request.CTX, use
 		cache_key += "_deleted"
 	}
 	if allowFromCache {
-		ids := make(map[string]string)
+		var ids model.StringMap
 		if err := s.rootStore.doStandardReadCache(s.rootStore.channelMembersForUserCache, cache_key, &ids); err == nil {
 			return ids, nil
 		}