test(core): add incremental convergence coverage

[qWaitCrypto]

test(core): add incremental convergence coverage

Summary

• Add convergence coverage for CocoIndex incremental updates by comparing the final materialized target snapshot after a multi-step incremental history against a fresh rebuild from the final source state.
• Use child targets and memoized document splitting to exercise the interaction between component memoization, child target cleanup, chunk cardinality changes, delete/reinsert, and stable path reuse.
• Cover three scenarios: multi-step convergence to fresh rebuild, delete-then-reinsert with the same memo key, and source iteration order changes not perturbing the final target snapshot.

Test plan

• uv run --group format ruff format --check python/tests/core/test_incremental_convergence.py
• uv run --group format ruff check python/tests/core/test_incremental_convergence.py
• env -u CONDA_PREFIX PATH="$HOME/.cargo/bin:$PATH" uv run pytest -q -p no:cacheprovider python/tests/core/test_incremental_convergence.py
• env -u CONDA_PREFIX PATH="$HOME/.cargo/bin:$PATH" uv run pytest -q -p no:cacheprovider python/tests/core/test_incremental_convergence.py python/tests/core/test_component_memo.py python/tests/core/test_component_target_states.py python/tests/core/test_function_memo.py