fix(release): unblock 0.0.8 release blocked by 100% coverage gate (#283)

## Problem

The **0.0.8 release never published**. The version-bump PR (#282) merged
to `main`, but the release itself is triggered manually via the
`release` workflow, and every run fails at the **`pre-release-checks` →
100% coverage gate** ([failing
run](https://github.com/langchain-ai/langchain-weaviate/actions/runs/29063814867)):

```
FAIL Required test coverage of 100.0% not reached. Total coverage: 99.52%
langchain_weaviate/_math.py   67   2   97%   19-20
```

Nothing on PyPI, no `libs/weaviate/v0.0.8` tag, no GitHub release.

## Root cause

`_math.py:19-20` is the **simsimd-success branch** of the backend
selection — it only runs when the optional `simsimd` binary is
importable:

```python
try:
    import simsimd
    _simsimd_available = True        # line 19
    _cdist_impl = simsimd.cdist      # line 20
except Exception:
    ... scipy / numpy fallback ...
```

- **Regular CI** (`_test.yml`) installs with `poetry install --with test
--extras simsimd`, so that branch runs → coverage hits 100% → passes.
- **The release job** (`_release.yml` `pre-release-checks`) installs
`--with test,test_integration` **without** the `simsimd` extra, so
simsimd is absent, lines 19-20 never run, and the gate can **never**
pass.

The `--cov-fail-under` was raised to 100% in #233, which also added
`_math.py` — so this has blocked releases since that landed; 0.0.8 is
the first release attempt after it.

## Fix

Two complementary changes:

1. **`_release.yml`** — add `--extras simsimd` to the
`pre-release-checks` install step so the release environment matches
regular CI.
2. **`_math.py`** — mark the environment-dependent simsimd-success
branch `# pragma: no cover`, so the coverage gate no longer depends on
whether an optional dependency is installed. The scipy/numpy fallback
branches stay genuinely covered via `test_math_simsimd_fallback.py`.

Either fix alone unblocks the release; together they make it robust
regardless of whether the optional native dependency is present.

## After merge

Re-run the `release` workflow (`working-directory: libs/weaviate`) from
`main` to publish 0.0.8 to PyPI and cut the tag/GitHub release.