[AI assisted]: MM-62295: Search and index archived channels as well. (#29796)

We add a new field delete_at in the channels template.

This field is then searched in the SearchChannels function.

Also added tests to verify that archived channels are searched
properly, and also indexed correctly.

https://mattermost.atlassian.net/browse/MM-62295

```release-note
- Now archived channels are searchable with ES/OS if TeamSettings.ExperimentalViewArchivedChannels is enabled.
- If there are old channels which were archived before a bulk index was run, users would need to purge indexes, and do bulk index again. Because those old archived channels are removed from the index when a bulk index is run.
```


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

server/channels/store/searchlayer/channel_layer.go

@@ -229,7 +229,7 @@ func (c *SearchChannelStore) AutocompleteInTeam(rctx request.CTX, teamID, userID
 }
 
 func (c *SearchChannelStore) searchAutocompleteChannels(engine searchengine.SearchEngineInterface, teamId, userID, term string, includeDeleted, isGuest bool) (model.ChannelList, error) {
-	channelIds, err := engine.SearchChannels(teamId, userID, term, isGuest)
+	channelIds, err := engine.SearchChannels(teamId, userID, term, isGuest, includeDeleted)
 	if err != nil {
 		return nil, err
 	}
@@ -247,7 +247,7 @@ func (c *SearchChannelStore) searchAutocompleteChannels(engine searchengine.Sear
 }
 
 func (c *SearchChannelStore) searchAutocompleteChannelsAllTeams(engine searchengine.SearchEngineInterface, userID, term string, includeDeleted, isGuest bool) (model.ChannelListWithTeamData, error) {
-	channelIds, err := engine.SearchChannels("", userID, term, isGuest)
+	channelIds, err := engine.SearchChannels("", userID, term, isGuest, includeDeleted)
 	if err != nil {
 		return nil, err
 	}