Tighten table and attachment annotation ref checks in tests

[FedyaBadyilo]

Summary

• Extend the existing _test_table_refs helper: reject dangling table/attach annotations always; require every table to be linked on placement formats (docx, pptx, pdf); optionally compare uid multisets so a lost ref is visible when two DOCX tables share a content-hash uid.
• Call the helper from the format tests that already parse those documents (docx, pptx, pdf, image-refs). Article and unused office media keep the weak side only (no dangling refs).
• Add a small unit test that builds a synthetic document, runs DefaultStructureExtractor + TreeConstructor, and checks both tables and attachments survive a raw_text merge.

This is tests only. It does not change the public contract and does not claim that every format must have a 1:1 annotation↔object map (HTML / Excel / CSV / email / uncited GROBID figures stay out).

Test plan

• python -m unittest tests.unit_tests.test_misc_object_refs -v
• API tests against a local dedoc on :1231:
  test_docx, test_pptx, test_structure_and_annotations,
  test_pdf_with_some_tables (scan + text layer),
  tests.api_tests.test_api_misc_with_images_refs
  (article needs Grobid; without it the request 500s in ArticleStructureExtractor, unrelated to this change)
• flake8 on the touched test files