Allow null query_cluster in datastore config to hide types

The acceptance tests demonstrate that setting `query_cluster: nil`
should hide index types from the GraphQL schema. However, loading
this configuration from YAML fails validation because the JSON schema
only accepts string values.

This commit updates the schema definitions to accept null values:
- Updated config_schema.yaml to allow ['string', 'null']
- Updated config.rb JSON schema to allow ["string", "null"]
- Updated RBS type signatures to allow String?
- Added regression test for YAML loading with null value
- Updated documentation to explain null behavior

The query_cluster field remains required to force explicit
configuration choices.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: anthonycastiglia-toast

elasticgraph-datastore_core/lib/elastic_graph/datastore_core/config.rb

@@ -132,9 +132,9 @@ class Config < Support::Config.define(:client_faraday_adapter, :clusters, :index
                 }],
                 properties: {
                   query_cluster: {
-                    type: "string",
-                    description: "Named search cluster to be used for queries on this index. The value must match be a key in the `clusters` map.",
-                    examples: ["main", "search_cluster"]
+                    type: ["string", "null"],
+                    description: "Named search cluster to be used for queries on this index. The value must match be a key in the `clusters` map. Set to `nil` to hide this index's types from the GraphQL schema.",
+                    examples: ["main", "search_cluster", nil]
                   },
                   index_into_clusters: {
                     type: "array",

elasticgraph-datastore_core/sig/elastic_graph/datastore_core/configuration/index_definition.rbs

@@ -3,15 +3,15 @@ module ElasticGraph
     module Configuration
       class IndexDefinitionSupertype
         attr_reader ignore_routing_values: ::Set[::String]
-        attr_reader query_cluster: ::String
+        attr_reader query_cluster: ::String?
         attr_reader index_into_clusters: ::Array[::String]
         attr_reader setting_overrides: ::Hash[::String, untyped]
         attr_reader setting_overrides_by_timestamp: ::Hash[::String, ::Hash[::String, untyped]]
         attr_reader custom_timestamp_ranges: ::Array[IndexDefinition::CustomTimestampRange]
 
         def initialize: (
           ignore_routing_values: ::Set[::String],
-          query_cluster: ::String,
+          query_cluster: ::String?,
           index_into_clusters: ::Array[::String],
           setting_overrides: ::Hash[::String, untyped],
           setting_overrides_by_timestamp: ::Hash[::String, ::Hash[::String, untyped]],
@@ -20,7 +20,7 @@ module ElasticGraph
 
         def with: (
           ?ignore_routing_values: ::Set[::String],
-          ?query_cluster: ::String,
+          ?query_cluster: ::String?,
           ?index_into_clusters: ::Array[::String],
           ?setting_overrides: ::Hash[::String, untyped],
           ?setting_overrides_by_timestamp: ::Hash[::String, ::Hash[::String, untyped]],

elasticgraph-datastore_core/spec/unit/elastic_graph/datastore_core/config_spec.rb

@@ -97,6 +97,43 @@ class DatastoreCore
         expect(config.max_client_retries).to eq 3
       end
 
+      it "allows `query_cluster: null` to hide index types from GraphQL schema" do
+        config = config_from_yaml(<<~YAML)
+          client_faraday_adapter: {}
+          clusters:
+            main1:
+              url: http://example.com/1234
+              backend: elasticsearch
+              settings:
+                foo: 23
+          index_definitions:
+            widgets:
+              query_cluster: null
+              index_into_clusters: ["main1"]
+              ignore_routing_values: []
+              setting_overrides: {}
+              setting_overrides_by_timestamp: {}
+              custom_timestamp_ranges: []
+        YAML
+
+        widgets_config = config.index_definitions.fetch("widgets")
+
+        # Verify the config was loaded correctly from YAML
+        expect(widgets_config).to eq(Configuration::IndexDefinition.new(
+          ignore_routing_values: [],
+          query_cluster: nil,
+          index_into_clusters: ["main1"],
+          setting_overrides: {},
+          setting_overrides_by_timestamp: {},
+          custom_timestamp_ranges: []
+        ))
+
+        # Verify the key property: query_cluster is nil (not a string, not missing)
+        # This nil value will cause accessible_from_queries? to return false,
+        # which hides the index's types from the GraphQL schema.
+        expect(widgets_config.query_cluster).to be_nil
+      end
+
       it "surfaces misspellings in `backend`" do
         expect {
           config_from_yaml(<<~YAML)

elasticgraph-local/lib/elastic_graph/local/spec_support/config_schema.yaml

@@ -141,12 +141,16 @@ properties:
                   number_of_shards: 32
             properties:
               query_cluster:
-                type: string
+                type:
+                - string
+                - 'null'
                 description: Named search cluster to be used for queries on this index.
-                  The value must match be a key in the `clusters` map.
+                  The value must match be a key in the `clusters` map. Set to `nil`
+                  to hide this index's types from the GraphQL schema.
                 examples:
                 - main
                 - search_cluster
+                -
               index_into_clusters:
                 type: array
                 items: