Update Combine method to treat warnings as failures and adjust tests (#37048)

Treat Commit Status Warnings as errors

> The root problem is that the definition of "warning" are different
across systems.
> 
> * Sometimes, "warning" is treated as "acceptable" (Gitea 1.25)
> * Sometimes, "warning" is mapped from "Result.UNSTABLE", which means
"there are test failures" and it is "failure" in Gitea
> 
> **To avoid breaking existing users, the best choice is to revert the
behavior on Gitea side: treat "warning" as "error".**


https://github.com/go-gitea/gitea/issues/37042#issuecomment-4158231611

fixes https://github.com/go-gitea/gitea/issues/37042

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Signed-off-by: Nicolas <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>