fix(xai): inject model_provider in response_metadata

[mdrxy]

plus tests minor rfc

[codspeed-hq]

CodSpeed Performance Report

Merging #33543 will not alter performance

_{Comparing mdrxy/xai-v1-tests (ec6a40e) with master (6997867)¹}

Summary

✅ 1 untouched
⏩ 33 skipped²

Footnotes

1. No successful run was found on master (8fd54f1) during the generation of this report, so 6997867 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 33 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[ccurme]

Suggested change

	@pytest.mark.parametrize("output_version", ["", "v1"])
	@pytest.mark.parametrize("output_version", ["v0", "v1"])

None also works for v0