Load the example scalar coercion adapter from a consistent path

The extension loader raises if the same extension constant is loaded from two
different `require_path`s within one process. `scalar_type_extension_spec`
loaded `ExampleScalarCoercionAdapter` via an absolute path while
`elasticgraph-schema_definition`'s `scalar_types_by_name_spec` used a
relative one, so any spec worker that ran both files failed 17 examples with
`InvalidExtensionError`. Use the same relative path (with a local copy of the
example adapter so the gem's own suite can resolve it).

Author: jwils

elasticgraph-json_ingestion/spec/support/example_extensions/scalar_coercion_adapter.rb

@@ -0,0 +1,15 @@
+# Copyright 2024 - 2026 Block, Inc.
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+#
+# frozen_string_literal: true
+
+class ExampleScalarCoercionAdapter
+  def self.coerce_input(value, ctx)
+  end
+
+  def self.coerce_result(value, ctx)
+  end
+end

elasticgraph-json_ingestion/spec/unit/elastic_graph/json_ingestion/schema_definition/schema_elements/scalar_type_extension_spec.rb

@@ -94,7 +94,10 @@ def grouping_missing_value_placeholder_for(mapping_type, **json_schema_options)
           end
 
           def scalar_coercion_adapter_path
-            ::File.join(CommonSpecHelpers::REPO_ROOT, "elasticgraph-schema_definition/spec/support/example_extensions/scalar_coercion_adapter")
+            # Must match the `defined_at` path used by other specs (e.g. in `elasticgraph-schema_definition`)
+            # that load this adapter: the extension loader raises if the same extension is loaded from two
+            # different paths within one process, as can happen when one worker runs both suites.
+            "support/example_extensions/scalar_coercion_adapter"
           end
 
           def build_api