cleanup_schema() doesn't drop functions, leaving public schema dirty

[affandar]

Problem

When using the public schema (the default), provider.cleanup_schema() leaves stored procedures behind. This is because:

1. cleanup_schema() calls the SQL cleanup_schema() function which only drops tables, not functions
2. The Rust code only runs DROP SCHEMA CASCADE for non-public schemas (as a safety measure)
3. Result: stored procedures accumulate in the public schema across test runs

Impact

• Tests using public schema may see interference from orphaned functions
• The public schema becomes "dirty" over time
• Function signature changes in migrations may fail because old function signatures still exist

Root Cause

The SQL cleanup_schema() function only drops tables:

CREATE OR REPLACE FUNCTION cleanup_schema()
RETURNS VOID AS $cleanup$
BEGIN
    DROP TABLE IF EXISTS instances CASCADE;
    DROP TABLE IF EXISTS executions CASCADE;
    -- ... only tables, no functions
END;
$cleanup$ LANGUAGE plpgsql;

Solution

Add DROP FUNCTION IF EXISTS for all stored procedures inside cleanup_schema().

Reference

Note: duroxide-pg-opt already has the correct pattern with function drops in its cleanup_schema().