[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/platform/services/searchengine/bleveengine/search.go

@@ -315,7 +315,7 @@ func (b *BleveEngine) IndexChannel(_ request.CTX, channel *model.Channel, userID
 	return nil
 }
 
-func (b *BleveEngine) SearchChannels(teamId, userID, term string, isGuest bool) ([]string, *model.AppError) {
+func (b *BleveEngine) SearchChannels(teamId, userID, term string, isGuest, _ bool) ([]string, *model.AppError) {
 	// This query essentially boils down to (if teamID is passed):
 	// match teamID == <>
 	// AND

server/platform/services/searchengine/interface.go

@@ -33,7 +33,7 @@ type SearchEngineInterface interface {
 	// IndexChannel indexes a given channel. The userIDs are only populated
 	// for private channels.
 	IndexChannel(rctx request.CTX, channel *model.Channel, userIDs, teamMemberIDs []string) *model.AppError
-	SearchChannels(teamId, userID, term string, isGuest bool) ([]string, *model.AppError)
+	SearchChannels(teamId, userID, term string, isGuest, includeDeleted bool) ([]string, *model.AppError)
 	DeleteChannel(channel *model.Channel) *model.AppError
 	IndexUser(rctx request.CTX, user *model.User, teamsIds, channelsIds []string) *model.AppError
 	SearchUsersInChannel(teamId, channelId string, restrictedToChannels []string, term string, options *model.UserSearchOptions) ([]string, []string, *model.AppError)

server/platform/services/searchengine/mocks/SearchEngineInterface.go

@@ -557,9 +557,9 @@ func (_m *SearchEngineInterface) RefreshIndexes(rctx request.CTX) *model.AppErro
 	return r0
 }
 
-// SearchChannels provides a mock function with given fields: teamId, userID, term, isGuest
-func (_m *SearchEngineInterface) SearchChannels(teamId string, userID string, term string, isGuest bool) ([]string, *model.AppError) {
-	ret := _m.Called(teamId, userID, term, isGuest)
+// SearchChannels provides a mock function with given fields: teamId, userID, term, isGuest, includeDeleted
+func (_m *SearchEngineInterface) SearchChannels(teamId string, userID string, term string, isGuest bool, includeDeleted bool) ([]string, *model.AppError) {
+	ret := _m.Called(teamId, userID, term, isGuest, includeDeleted)
 
 	if len(ret) == 0 {
 		panic("no return value specified for SearchChannels")
@@ -567,19 +567,19 @@ func (_m *SearchEngineInterface) SearchChannels(teamId string, userID string, te
 
 	var r0 []string
 	var r1 *model.AppError
-	if rf, ok := ret.Get(0).(func(string, string, string, bool) ([]string, *model.AppError)); ok {
-		return rf(teamId, userID, term, isGuest)
+	if rf, ok := ret.Get(0).(func(string, string, string, bool, bool) ([]string, *model.AppError)); ok {
+		return rf(teamId, userID, term, isGuest, includeDeleted)
 	}
-	if rf, ok := ret.Get(0).(func(string, string, string, bool) []string); ok {
-		r0 = rf(teamId, userID, term, isGuest)
+	if rf, ok := ret.Get(0).(func(string, string, string, bool, bool) []string); ok {
+		r0 = rf(teamId, userID, term, isGuest, includeDeleted)
 	} else {
 		if ret.Get(0) != nil {
 			r0 = ret.Get(0).([]string)
 		}
 	}
 
-	if rf, ok := ret.Get(1).(func(string, string, string, bool) *model.AppError); ok {
-		r1 = rf(teamId, userID, term, isGuest)
+	if rf, ok := ret.Get(1).(func(string, string, string, bool, bool) *model.AppError); ok {
+		r1 = rf(teamId, userID, term, isGuest, includeDeleted)
 	} else {
 		if ret.Get(1) != nil {
 			r1 = ret.Get(1).(*model.AppError)