fix: isolate Table elements in pre-chunks (#4307)
## Summary
This change enforces the documented table-isolation guarantees in
chunking:
- Table and TableChunk are always staged in their own pre-chunk and
never combined with adjacent non-table elements into a CompositeElement.
- PreChunkCombiner will not merge pre-chunks when either side contains a
table-family element, preventing “table gets wrapped/merged” behavior
when combine_text_under_n_chars is enabled.
- Shared helper functions centralize the table-isolation checks in
unstructured.chunking.base.
Also includes:
- Updated/adjusted chunking tests to reflect the new behavior.
- Added a dedicated test_table_isolation.py regression suite.
- Version bump + CHANGELOG.md entry to document the fix.
Closes #3921
