Migrate JSON ingestion specs into gem

[jwils]

Why

The JSON ingestion gem should own the JSON-schema tests that cover its extracted implementation so the gem can enforce full coverage without a SimpleCov skip.

What

• Move JSON schema generation, metadata merge, pruner, and matcher specs into elasticgraph-json_ingestion.
• Add JSON-ingestion artifact-manager integration coverage for versioned schema artifacts, version-bump enforcement, metadata refresh, and schema evolution diagnostics.
• Add focused wrapper/scalar specs for the remaining extracted JSON-ingestion paths.
• Remove the temporary SimpleCov filter for elasticgraph-json_ingestion.

Verification

• script/run_gem_specs elasticgraph-json_ingestion
• script/type_check elasticgraph-json_ingestion
• BUNDLE_GEMFILE=Gemfile bundle exec rspec --format progress spec/integration/elastic_graph/schema_definition/rake_tasks_spec.rb spec/unit/elastic_graph/schema_definition/runtime_metadata/scalar_types_by_name_spec.rb from elasticgraph-schema_definition/
• bundle exec standardrb ... on touched specs/support

Full script/run_gem_specs elasticgraph-schema_definition was blocked locally because Docker is not running, so the test datastore could not be booted.

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