fix: fall back to pypdfium2 page count when docling-parse returns -1

[majiayu000]

Summary

Fixes #3031

When docling-parse's C++ backend fails to parse a PDF's page tree (QPDF exception), number_of_pages() returns -1. The current page_count() method returns this value directly, causing is_valid() to reject documents that pypdfium2 can read fine.

Changes

• Added fallback in page_count(): when docling-parse returns a negative count, fall back to pypdfium2's page count (len(self._pdoc)) with a warning log
• Added test_page_count_fallback_on_parse_failure test that mocks docling-parse failure and verifies pypdfium2 fallback

Test plan

• pre-commit run --all-files passes (Ruff + MyPy)
• All 5 existing + new backend tests pass
• New test verifies page_count() returns pypdfium2 count when docling-parse returns -1
• New test verifies is_valid() returns True in fallback scenario

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[dolfim-ibm]

Duplicate of #3040

[majiayu000]

Close for dup