fix: allow ts_active_atoms to accept list[int] directly (closes #233)

[botbikamordehai2-sketch]

What

The ts_active_atoms field in BlockGeom was typed as Hybrid, which requires a mode string and cannot be instantiated with a plain list. According to ORCA documentation, ts_active_atoms expects a simple list of integers, not a wrapped Hybrid object.

Fix

Changed the type of ts_active_atoms from Hybrid to Optional[list[int]] (with appropriate pydantic validation to ensure positive integers). Removed the Hybrid dependency for this field, allowing users to pass [1, 2, 3] directly.

Closes #233

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