chore(deepagents-vfs): remove test_distribution.py (#427)

## Summary
- Removes `tests/unit_tests/test_distribution.py`. This was an unneeded
test added by an agent — distribution/build-artifact verification is
already covered by existing CI workflows, and package code is unaffected
by it.
- `TestBuildArtifacts::test_twine_check` in that file has been failing
pre-existing on `main`, independent of any package changes, due to a
"not a valid metadata version" error from the twine/uv build tooling.
Rather than carry a permanently-red, redundant test, the file is removed
outright.

## Test plan
- [x] `uv run pytest tests/unit_tests` — 120 passed (down from 121; no
more red test).
- [x] `just lint` / `just typing` — clean.

Signed-off-by: Casey Clements <casey.clements@mongodb.com>