Fix empty-string coercion for INT/FLOAT optional inputs in AcestepCPPGenerate

[Copilot]

Older ComfyUI workflows serialize unset optional widget values as "" instead of their numeric defaults, causing runtime failures when Python tries to cast them: int("") and float("") both raise.

Changes

• nodes.py: Added _coerce_int(value, default) helper mirroring the existing _coerce_float; extended the coercion block in AcestepCPPGenerate.generate() to cover the three missing fields:

  lm_top_k = _coerce_int(lm_top_k, 0)
  repainting_start = _coerce_float(repainting_start, -1.0)
  repainting_end = _coerce_float(repainting_end, -1.0)

• tests/test_nodes.py: Added TestCoerceInt covering passthrough, float truncation, valid string, empty string, and whitespace cases.

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