[CI] Expand test_guided_generate to test all backends

[mgoin]

Previously we were only testing the default structured output backend, so we should expand to explicitly call all backends. This revealed some lack of support in outlines for json_object and lm-format-enforcer for regex patterns in json schemas.

I also found some of the tests are flaky for some backends due to bad model output (many many newlines or filler tokens), so using a higher quality Qwen2.5 model helped

[github-actions]

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