refactor: extract ES-compat handler logic into reusable functions

[congx4]

Summary

• Extract JSON construction from warp handlers into standalone pub(crate) functions callable without HTTP
• Enable non-HTTP consumers (e.g., gRPC passthrough) to reuse ES-compatible logic directly
• Make model and rest_handler modules pub(crate) in mod.rs

Motivation

Downstream consumers need to call ES-compatible handler logic directly (without a network round-trip through warp). This refactoring extracts the core logic into reusable functions while keeping existing HTTP handlers working identically.

Changes

Extracted new functions:

• es_compat_cluster_info — cluster info JSON
• es_compat_nodes_info — nodes info JSON
• es_compat_search_shards — search shards JSON
• es_compat_aliases — empty aliases object
• es_compat_cluster_health_check — cluster health check
• es_compat_delete_scroll — scroll delete response

Made pub(crate): es_compat_index_mapping, es_compat_index_search, es_compat_index_count, es_compat_index_stats, es_compat_cat_indices, es_compat_index_cat_indices, es_compat_resolve_index, es_compat_index_field_capabilities, es_compat_index_multi_search, es_scroll, ElasticsearchCountResponse

Test plan

• All 64 existing ES API tests pass
• cargo +nightly fmt clean
• No behavior changes — pure refactoring

🤖 Generated with Claude Code