## Summary

- Move OpenAI-compatible `/v1/vector_stores*` routes from the `VectorIO`
OpenAPI tag to a top-level `Vector Stores` tag.
- Keep `/v1/vector-io/*` insert/query routes under `VectorIO`.
- Add the `Vector Stores` tag to the legacy API tag ordering/grouping
and regenerate the OpenAPI/Stainless specs.
- Add a focused unit test that locks the generated FastAPI OpenAPI tags
for both route families.

## Root cause

The vector store routes are implemented in the VectorIO FastAPI router
and inherited the router-level `VectorIO` tag, so generated API docs
placed the OpenAI-compatible Vector Stores surface under VectorIO.

## Review

Pre-landing review found no issues. Scope is limited to route tags,
generated specs, and the focused regression test.

## Validation

- `PATH=/Users/farceo/.local/bin:$PATH uv run --python 3.12 pytest
tests/unit/server/test_vector_io_fastapi_router.py -q`
- `PATH=/Users/farceo/.local/bin:$PATH uv run --python 3.12
./scripts/run_openapi_generator.sh`
- `git diff --check`
- `cd docs && npm run gen-api-docs all`
- `cd docs && npm run build`

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>