## Summary
- Revert the 2.6 nullable vector support backport from #49520.
- Restore the previous 2.6 behavior while the nullable vector support is
not ready on this branch.
- Format the changed Python test files and add file-level Ruff
suppressions for legacy star-import tests touched by the revert.

pr: #46305
issue: #49470

## Test plan
- [x] `ruff check $(git diff --name-only --diff-filter=ACMR
upstream/2.6...HEAD -- tests/**/*.py)`
- [x] `ruff format --check --diff $(git diff --name-only
--diff-filter=ACMR upstream/2.6...HEAD -- tests/**/*.py)`
- [x] `git diff --check`

Signed-off-by: marcelo-cjl <marcelo.chen@zilliz.com>