fix(ocr): support Latin languages with RapidOCR ONNX

[Success6666]

Summary

• use PP-OCRv4 mobile models when the default ONNX path receives a language that resolves to the generic latin model set
• keep the existing PP-OCRv6 behavior for English, Chinese, and explicitly configured model paths
• add regression coverage for the generated RapidOCR parameters and update the language option description

Fixes #3840

Tests

• python -m pytest tests/test_rapid_ocr_lang.py tests/test_rapid_ocr_model.py -q (19 passed)
• python -m ruff check docling/models/stages/ocr/rapid_ocr_model.py docling/datamodel/pipeline_options.py tests/test_rapid_ocr_lang.py
• python -m ruff format --check docling/models/stages/ocr/rapid_ocr_model.py docling/datamodel/pipeline_options.py tests/test_rapid_ocr_lang.py
• verified with RapidOCR 3.9.1 and ONNX Runtime 1.23.2 that lang=[es] initializes successfully and runs inference on a synthetic Spanish text image
• verified the default Chinese and English ONNX initialization paths remain successful

Checklist

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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