fix: allow curl fetcher to properly fallback to next mirror

[myml]

The current ostree curl fetcher has two issues that prevent proper fallback when using a mirrorlist:

1. The max_size check does not account for non-2xx responses. When a server returns 3xx, 4xx, or 5xx, the response body may exceed max_size.
2. Some state variables and temporary files are not reset when falling back to the next mirror.

[openshift-ci]

Hi @myml. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[gemini-code-assist]

Code Review

This pull request effectively addresses two issues with the curl fetcher's mirror fallback logic. The first change correctly prevents max_size checks from failing on non-2xx HTTP responses, which could have large bodies (e.g., error pages). The second change properly resets request state variables and temporary files before retrying with a new mirror. I have one suggestion to improve the robustness of the state reset logic by adding error handling for file operations.