feat(extraction): add Granite Vision 4.1 as alternative KVP extraction model (#3398)

Introduce a unified TransformersExtractionModel that supports multiple
prompt styles via an ExtractionPromptStyle enum. This replaces the
need for separate model classes per VLM.

- Add ExtractionPromptStyle enum (NUEXTRACT, GRANITE_VISION)
- Add prompt_utils.py with style-specific prompt builders
- Add TransformersExtractionModel with prompt-style dispatch
- Add GRANITE_VISION_4_1_TRANSFORMERS model spec
- Add extraction_prompt_style field to VlmExtractionPipelineOptions

Signed-off-by: Ben Wiesel <benwiesel@ibm.com>
Co-authored-by: Ben Wiesel <benwiesel@ibm.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>