DuckDB bugbash: Python model parity

## Goal
Compare dbt Core `dbt-duckdb` vs dbt Fusion DuckDB behavior for Python models.

## Experiment
Cover at least:
- a simple pandas-returning Python model
- a Python model that reads upstream relations via `dbt.ref()` / DuckDB relation APIs

## Why this matters
Python models are a headline `dbt-duckdb` feature and show up in public DuckDB+dbt projects.

## Success criteria
- Equivalent Python models execute successfully in both engines, or
- a minimal repro isolates the Fusion parity gap

## Notes
Exclude static-analysis / `SQL_understanding` concerns from scope.