feat: add HTML pre-heading content layer option

[xdong99]

Summary

• Add HTMLBackendOptions.pre_heading_content_layer so callers can choose the content layer assigned to HTML content before the first heading.
• Keep the existing inferred furniture behavior as the default when the option is unset.
• Add lightweight HTML backend tests covering the default behavior and explicit ContentLayer.BODY override.

Why

For HTML fragments used in RAG pipelines, content before the first heading may be semantically part of the body. The new option lets callers preserve that content as body text without disabling the existing default furniture inference for everyone.

Fixes #2487.

Checks

• python -m pytest tests/test_backend_html_content_layer.py -q
• python -m ruff check docling/datamodel/backend_options.py docling/backend/html_backend.py tests/test_backend_html_content_layer.py
• python -m ruff format --check docling/datamodel/backend_options.py docling/backend/html_backend.py tests/test_backend_html_content_layer.py

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