[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/enterprise/elasticsearch/common/indexing_job_test.go

@@ -0,0 +1,64 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.enterprise for license information.
+
+package common
+
+import (
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
+	"github.com/mattermost/mattermost/server/v8/channels/store/storetest/mocks"
+)
+
+func TestBulkIndexChannelsWithDeletedChannels(t *testing.T) {
+	// Create test channels - one active, one deleted
+	activeChannel := &model.Channel{
+		Id:       "ch1",
+		Type:     model.ChannelTypeOpen,
+		DeleteAt: 0,
+	}
+	deletedChannel := &model.Channel{
+		Id:       "ch2",
+		Type:     model.ChannelTypeOpen,
+		DeleteAt: 123456,
+	}
+	channels := []*model.Channel{activeChannel, deletedChannel}
+
+	// Mock store
+	mockStore := &mocks.Store{}
+	mockChannelStore := &mocks.ChannelStore{}
+	mockStore.On("Channel").Return(mockChannelStore)
+	defer mockStore.AssertExpectations(t)
+
+	// Since these are open channels, GetAllChannelMemberIdsByChannelId won't be called
+	// But GetTeamMembersForChannel will be called for both channels
+	mockChannelStore.On("GetTeamMembersForChannel", "ch1").Return([]string{"team1"}, nil)
+	mockChannelStore.On("GetTeamMembersForChannel", "ch2").Return([]string{"team1"}, nil)
+
+	// Track which channels were actually indexed
+	indexedChannels := make(map[string]bool)
+
+	// Mock bulk processor function
+	addItemToBulkProcessorFn := func(_, op, id string, _ io.ReadSeeker) error {
+		assert.Equal(t, indexOp, op) // Should always be index, not delete
+		indexedChannels[id] = true
+		return nil
+	}
+
+	config := &model.Config{}
+	config.ElasticsearchSettings.IndexPrefix = model.NewPointer("test_")
+
+	// Call the function
+	lastChannel, appErr := BulkIndexChannels(config, mockStore, mlog.CreateConsoleTestLogger(t), addItemToBulkProcessorFn, channels, IndexingProgress{})
+
+	// Verify results
+	require.Nil(t, appErr)
+	assert.Equal(t, deletedChannel, lastChannel)
+	assert.True(t, indexedChannels["ch1"], "Active channel should be indexed")
+	assert.True(t, indexedChannels["ch2"], "Deleted channel should also be indexed")
+}