Summary

• Move mapping-removal noise handling into elasticgraph-admin so all admin users benefit.
• Preserve current mapping fields in concrete index update payloads and diffs, hiding unsupported field removals while still showing new fields and mapping parameter changes.
• Remove deleted mapping fields from index templates so future concrete indices do not inherit fields that no longer exist in the schema.
• Keep settings diffs complete, and report new index templates as created instead of dumping the full config diff.

Behavioral change: index templates now drop removed fields

This is more than an output change. Previously, merge_properties preserved existing template fields forever (a guard added after a near-SEV caused by dropping a template field while deployed indexers still published it). This PR removes that guard: templates are now written to exactly match the schema, so removed fields are dropped and stale fields no longer accumulate.

The operational caveat (now documented in ForIndexTemplate#put_index_template): new rollover indices are auto-created from the template at indexing time with dynamic: strict mappings, so a field must only be removed from the schema once no deployed indexer still publishes it (and no old events containing it will be replayed). Dropped fields appear as deletions in the reported template diff, giving operators a chance to catch a premature removal. Concrete indices are unaffected — their existing fields are always preserved, since the datastore doesn't support removing index mapping fields.

Context

Mapping removals are unsupported by Elasticsearch/OpenSearch concrete indices, so showing removed fields every admin run just makes the diffs harder to read. Index templates can be rewritten without those fields, so this keeps existing concrete indices stable while allowing future indices to reflect the current schema. This addresses #766 and the review feedback to improve elasticgraph-admin directly instead of adding an elasticgraph-admin_lambda opt-in.

Verification

• script/lint
• script/spellcheck
• script/type_check
• script/quick_build
• Full CI (including datastore-backed integration/acceptance specs) is green.