Parent: #3401

Summary

Index metadata APIs should expose canonical full field paths for nested fields, not leaf names recovered from field ids.

Scope

• Update list_indices() / IndexConfig.columns to return canonical field paths.
• Ensure index_stats() and related table/index metadata APIs do not lose parent path information.
• Avoid ambiguity when multiple nested fields share the same leaf name.
• Preserve local and remote response compatibility where possible, but make the nested-field contract explicit.

Acceptance criteria

• An index on metadata.user_id is listed as metadata.user_id, not user_id.
• A top-level user_id and nested metadata.user_id can both be listed without ambiguity.
• Escaped paths round-trip in list/describe/stats APIs.