Add database transaction wrappers and plan helpers

[NikolayS]

• Add postgres_ai schema for organizing our objects
• Move pg_statistic view from public to postgres_ai schema
• Add postgres_ai.explain_generic() function (SECURITY DEFINER) for collecting generic query plans with optional HypoPG index testing
• Update search_path to include postgres_ai first
• Change transaction model to wrap each step in its own begin/commit instead of grouping non-optional steps in a single transaction
• Update verification to check for postgres_ai schema and new function

Relates to: https://gitlab.com/postgres-ai/postgres_ai/-/issues/68

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Note

Adds foundational schema and helpers plus safer initialization flow.

• Creates postgres_ai schema; moves pg_statistic view to postgres_ai.pg_statistic and grants SELECT
• Adds SECURITY DEFINER functions: postgres_ai.explain_generic(text, text, text) and postgres_ai.table_describe(text) with EXECUTE grants
• Sets monitoring role search_path to postgres_ai, "$user", public, pg_catalog
• Refactors init to execute each plan step in its own begin/commit (with rollback on failure), including optional steps
• Extends verification to require USAGE on postgres_ai, presence/privileges on postgres_ai.pg_statistic, search_path containing postgres_ai, and EXECUTE on the new functions
• Updates/extends tests for the new schema, functions, and per-step transaction behavior

^{Written by Cursor Bugbot for commit 45d78a4. Configure here.}

[NikolayS]

@cursor review

[cursor]

✅ Bugbot reviewed your changes and found no bugs!