[BUG] Upload failed: {"message":"Token required because branch is protected"} is unhelpful

Describe the bug
I'm trying to get tokenless to work in forks. And all I get is: https://github.com/check-spelling-sandbox/codecov-action/actions/runs/22902897563/job/66453728930#step:5:418

debug - 2026-03-10 12:40:22,886 -- Upload result --- {"result": "RequestResult(error=RequestError(code='HTTP Error 400', params={}, description='{\"message\":\"Token required because branch is protected\"}\\n'), warnings=[], status_code=400, text='{\"message\":\"Token required because branch is protected\"}\\n')"}
error - 2026-03-10 12:40:22,886 -- Upload failed: {"message":"Token required because branch is protected"}

To Reproduce
Steps to reproduce the behavior:

1. Make some small changes to the codecov action to try to get tokenless to behave: https://github.com/check-spelling-sandbox/codecov-action/commit/7eaa93276766b8789b834df71e1ca5cb9ad9f9f1
2. https://github.com/check-spelling-sandbox/codecov-action/actions/runs/22902897563/job/66453728930#step:5:418

Expected behavior
Either:

• A clear and concise error explaining what it's talking about. I promise that I did not set up a codecov account and mark the use-tokenless-for-fork branch as protected
• It should work (which is to say, someone explains what additional changes I need to do to make this actually work at which time we can fix the branch and make a PR to this repository to fix the use case)

Regression
No

Screenshots

Product Area
reporting

Versions

• OS: ubuntu@24
• Git Host: GitHub
• CI/CD: GitHub Actions
• Uploader: codecov-action@7eaa93276766b8789b834df71e1ca5cb9ad9f9f1

Commit and CI link
https://github.com/check-spelling-sandbox/codecov-action/commit/7eaa93276766b8789b834df71e1ca5cb9ad9f9f1
https://github.com/check-spelling-sandbox/codecov-action/actions/runs/22902897563/job/66453728930

Additional context

There are probably a dozen bugs about tokenless and forks. Periodically I come and complain that it's a mess.

I'm currently here because dragonflyoss uses codecov and forks of their repository trigger codecov uploads which fail which results in every workflow failing: https://github.com/check-spelling-sandbox/nydus/actions/workflows/smoke.yml -- some of these fail exclusively because of codecov, and some would also fail because of some change I made. But because they always fail because of codecov, I can't easily see when I broke something. This defeats the point of having a smoketest or ci.

Start by reproducing the failure using the referenced codecov-action commit and GitHub Actions job, then inspect the uploader path that reports the HTTP 400 response. Compare tokenless uploads from forks with protected-branch behavior. Done means either tokenless fork uploads work or the action reports a clear, actionable explanation of the required configuration.