fix: extra seed without foreign key

[michdr]

Note

Handle missing foreign keys during extra seed extraction and add an E2E probe test for seeds with extra columns.

• Fix (local generation):
  • Safely handle missing foreign_keys by iterating over (tgt_g_table.foreign_keys or []) when extracting context FK for extra seed columns in mostlyai/sdk/_local/execution/step_generate_data.py.
• Tests:
  • Extend end-to-end test to probe with seed containing extra columns (e.g., {"a": ["a1"], "x": ["x"]}) in tests/_local/end_to_end/test_simple_flat.py.

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