fix: isolate VLM pipeline options to avoid cache collisions

[serge-medvedev]

• Prevent cached Standard PdfFormatOption from being reused for VLM tasks by hashing pipeline cls/backend and pipeline options type, ensuring VLM requests get their own PdfFormatOption and the right pipeline class (avoids the .ocr_options crash in Docling Serve when hybrid chunking sends image payloads with pipeline: vlm and a VLM API model).
• Keep VLM runs aligned with picture description settings (classification/description flags, API/local config, area threshold) and
  use the VLM image format option when the VLM pipeline is selected.
• Tests: pytest tests/test_options.py

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