feat: Canonicalize the OCR languages across all engines according to the BCP-47 standard

[nikos-livathinos]

This PR is an implementation of the discussion #3915

Checklist:

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

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[codecov]

Codecov Report

❌ Patch coverage is 81.67808% with 107 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/models/stages/ocr/ocr_mac_model.py	25.92%	20 Missing ⚠️
docling/models/stages/ocr/tesseract_utils.py	70.49%	9 Missing and 9 partials ⚠️
docling/utils/ocr_language.py	93.29%	8 Missing and 5 partials ⚠️
docling/models/stages/ocr/nemotron_ocr_model.py	42.85%	8 Missing ⚠️
docling/models/stages/ocr/tesseract_ocr_model.py	65.21%	5 Missing and 3 partials ⚠️
docling/cli/remote.py	30.00%	7 Missing ⚠️
docling/models/stages/ocr/auto_ocr_model.py	69.56%	6 Missing and 1 partial ⚠️
docling/models/stages/ocr/easyocr_model.py	85.71%	4 Missing and 3 partials ⚠️
docling/models/base_ocr_model.py	81.48%	4 Missing and 1 partial ⚠️
docling/models/stages/ocr/kserve_v2_ocr_model.py	76.47%	3 Missing and 1 partial ⚠️
... and 4 more

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