Improve admin mapping change output

[jwils]

🤖 Updated by my AI agent.

Summary

• Move mapping-removal noise handling into elasticgraph-admin so all admin users benefit.
• Preserve current mapping fields in update payloads and diffs, hiding unsupported field removals while still showing new fields and mapping parameter changes.
• Keep settings diffs complete, and report new index templates as created instead of dumping the full config diff.

Context

Mapping removals are unsupported by Elasticsearch/OpenSearch, so showing removed fields every admin run just makes the diffs harder to read. This addresses #766 and the review feedback to improve elasticgraph-admin directly instead of adding an elasticgraph-admin_lambda opt-in.

Verification

• script/lint --fix
• script/spellcheck -w
• script/type_check
• bundle exec rspec elasticgraph-admin/spec/unit
• bundle exec rspec elasticgraph-admin/spec/unit/elastic_graph/admin/index_definition_configurator/mapping_update_spec.rb
• bundle exec rspec elasticgraph-local/spec/unit
• script/run_gem_specs elasticgraph-admin_lambda

Datastore-backed specs were attempted locally but blocked because the datastore is not running and this environment has neither docker nor docker-compose available to boot it. GitHub Actions is running those datastore-backed checks.

[myronmarston]

I'd like to see the output of the admin process improved (it's why I opened #766!) but:

• I don't think it needs to be an opt-in thing. I don't think we've ever found the delete changes to be useful and I doubt anyone else has. It's simpler to just improve it for everyone with no opt-in needed.
• Can we improve the output in elasticgraph-admin so it improves the output for projects which don't use elasticgraph-admin_lambda?