Remove remaining JSON schema usage from schema definition specs

[jwils]

Why

On #1224, @myronmarston noted that mentions of "JSON schema" in elasticgraph-schema_definition/spec should approach zero as part of the extraction, but his grep showed many remained.

What

• The graphql_schema, datastore_config, and runtime_metadata spec supports now define schemas with no extension modules, and the json_schema calls that existed only to satisfy the JSON ingestion extension's scalar validation are removed.
• The scalar "requires json_schema" test (duplicated by the json_ingestion suite) is deleted, and the long/unsigned_long placeholder-inference tests that depend on JSON schema bounds moved to scalar_type_extension_spec in elasticgraph-json_ingestion (along with a built-in-scalar placeholder map test, since JsonSafeLong's placeholder differs with the extension loaded).
• The reserved-type-name test now exercises the core reserved_type_names mechanism directly; the ElasticGraphEventEnvelope reservation is already covered by the json_ingestion suite.
• rake_tasks_spec runs its synthetic schemas without the extension and no longer asserts on JSON schema artifacts (covered by json_ingestion's schema_artifact_manager_extension_spec). A new short-diff test keeps SchemaArtifactManager#truncate_diff fully covered.
• The "different JSON schema between interface and object fields" implements test moved to json_ingestion's json_schema_spec.

What intentionally remains: the rake-task tests that evaluate the repo's own config/schema.rb keep the extension (that schema is a JSON ingestion application), the define_schema test-support seam tests (the seam exists for optional ingestion extensions), and documentation text about JSON-the-format (JsonSafeLong etc.).

Verification

• script/run_gem_specs elasticgraph-schema_definition and script/run_gem_specs elasticgraph-json_ingestion (both 100% line + branch coverage)
• script/type_check
• script/quick_build

Stack

Current PR is marked with ->.

• #1199 Add JSON ingestion gem shell
• #1200 Add schema definition extension plumbing
• #1201 Move JSON schema helpers to json ingestion gem
• #1203 Namespace JSON schema helpers under JSON ingestion
• #1204 Add JSON ingestion indexing extensions
• #1215 Wire JSON ingestion schema extension modules
• #1205 Use JSON ingestion as schema definition extension
• #1202 Document JSON ingestion schema APIs
• #1224 Migrate JSON ingestion specs into gem
• #1220 Add configurable indexing event decoder
• #1249 Load the JSON ingestion spec extension default lazily
• -> #1250 Remove remaining JSON schema usage from schema definition specs