deps: fix pytest and pygments Dependabot alerts (#284)

## Summary

Fixes the two open Dependabot alerts in `libs/weaviate/poetry.lock`:

- GHSA-6w46-j5rx-g56g / CVE-2025-71176: bumps `pytest` from 8.4.2 to
9.1.1 (patched range is >=9.0.3)
- GHSA-5239-wwwm-4pmq / CVE-2026-4539: bumps `Pygments` from 2.19.2 to
2.20.0

Because `pytest` 9.x requires compatible test plugins, this also updates
test-only bounds/lock entries for:

- `pytest-asyncio` 0.23.8 -> 1.4.0
- `syrupy` 4.9.1 -> 5.5.3
- adds `backports-asyncio-runner` for Python 3.10 via `pytest-asyncio`

Scoped to the Weaviate package test/development dependency metadata and
lockfile.

## Validation

- `poetry check --lock --directory
/tmp/langchain-weaviate/libs/weaviate`
- Verified lockfile versions: `pytest==9.1.1`, `Pygments==2.20.0`,
`pytest-asyncio==1.4.0`, `syrupy==5.5.3`

## Notes

- Did not merge; opening as draft for CI/human review.

Co-authored-by: Langster Security Agent <john@langchain.dev>