MM-63298: [AI assisted] Elasticsearch add a global search prefix (#30417)

This PR adds functionality to search by a global search prefix.
This allows Mattermost to be used across multiple data centers
with multiple Elasticsearch instances synchronized using
cross-cluster replication.

While here, we also add tests cases to cover for some missing
search interface methods.

For now, no system console setting is exposed. Because IndexPrefix
is also not exposed. It can be added later if a need arises.

https://mattermost.atlassian.net/browse/MM-63298

```release-note
A new config setting ElasticsearchSettings.GlobalSearchPrefix is added
which can be used to search across multiple indices having a common prefix.
This is useful in a scenario with multiple Elasticsearch instances, where
multiple instances are writing to different indices with different prefixes
using the ElasticsearchSettings.IndexPrefix setting.
```

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

server/public/model/config.go

@@ -2916,6 +2916,7 @@ type ElasticsearchSettings struct {
 	AggregatePostsAfterDays       *int    `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"` // telemetry: none
 	PostsAggregatorJobStartTime   *string `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"` // telemetry: none
 	IndexPrefix                   *string `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
+	GlobalSearchPrefix            *string `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
 	LiveIndexingBatchSize         *int    `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
 	BulkIndexingTimeWindowSeconds *int    `json:",omitempty"` // telemetry: none
 	BatchSize                     *int    `access:"environment_elasticsearch,write_restrictable,cloud_restrictable"`
@@ -3009,6 +3010,10 @@ func (s *ElasticsearchSettings) SetDefaults() {
 		s.IndexPrefix = NewPointer(ElasticsearchSettingsDefaultIndexPrefix)
 	}
 
+	if s.GlobalSearchPrefix == nil {
+		s.GlobalSearchPrefix = NewPointer("")
+	}
+
 	if s.LiveIndexingBatchSize == nil {
 		s.LiveIndexingBatchSize = NewPointer(ElasticsearchSettingsDefaultLiveIndexingBatchSize)
 	}
@@ -4427,6 +4432,16 @@ func (s *ElasticsearchSettings) isValid() *AppError {
 		return NewAppError("Config.IsValid", "model.config.is_valid.elastic_search.invalid_backend.app_error", nil, "", http.StatusBadRequest)
 	}
 
+	if *s.GlobalSearchPrefix != "" && *s.IndexPrefix == "" {
+		return NewAppError("Config.IsValid", "model.config.is_valid.elastic_search.empty_index_prefix.app_error", nil, "", http.StatusBadRequest)
+	}
+
+	if *s.GlobalSearchPrefix != "" && *s.IndexPrefix != "" {
+		if !strings.HasPrefix(*s.IndexPrefix, *s.GlobalSearchPrefix) {
+			return NewAppError("Config.IsValid", "model.config.is_valid.elastic_search.incorrect_search_prefix.app_error", map[string]any{"IndexPrefix": *s.IndexPrefix, "GlobalSearchPrefix": *s.GlobalSearchPrefix}, "", http.StatusBadRequest)
+		}
+	}
+
 	return nil
 }