### What this PR does / why we need it?
Add test for `outlines` backend for structured output in CI.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Tests have all passed with:
```bash
pytest -sv tests/e2e/singlecard/test_guided_decoding.py
```
- vLLM version: v0.10.0
- vLLM main:
53415653ff
---------
Signed-off-by: shen-shanshan <467638484@qq.com>