FIX: reject unsupported demo datasets in export_code

[biru-codeastromer]

Summary

• reject unsupported supervised demo datasets when export_code is asked to include a forecasting fit example
• keep valid forecasting datasets like airline working as before
• add focused regression tests for supported and unsupported dataset paths

Why

export_code_tool(..., include_fit_example=True, dataset=...) currently trusts the raw discovered demo dataset map and can generate misleading forecasting example code for datasets like basic_motions.

Testing

• .venv/bin/ruff check .
• .venv/bin/ruff format --check .
• .venv/bin/pytest tests/test_codegen.py -q
• .venv/bin/pytest -q

Closes #388