[BUG] Version 11.2.6 does not detect --auto-load-params-from ...

Describe the bug
This is a real bug reported and seen by me (not a AI generated slop) in our Nix CI where we use codecov upload-coverage ....

This bug made me pin to 10.4.0 for a long time (with Nix) because the 11.2.6 does not work:

• The 10.4.0 binary does handle --auto-load-params-from githubactions fine.
• The binary 11.2.6 does not, all loaded values are null (same environment)

I used Claude Opus 4.8 debugging showed me that the problem seems to be in codecov click dependency.
Could somebody confirm that this is the root cause?

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CodecovOption CI-adapter fallback broken with click ≥ 8.3 (--slug not auto-loaded → "The provided slug is invalid")

Root cause

click 8.3 introduced an UNSET sentinel: click.Parameter.get_default() now returns UNSET (not None) when an option has no explicit default.

CodecovOption.get_default in codecov_cli/fallbacks.py only guards against None:

def get_default(self, ctx, call=True):
    res = super().get_default(ctx, call=call)
    if res is not None:          # UNSET is not None -> returns here
        return res
    if self.fallback_field is not None:
        if ctx.obj.get("ci_adapter") is not None:
            res = ctx.obj.get("ci_adapter").get_fallback_value(self.fallback_field)
            ...

Summary

The CI-adapter fallback for CodecovOption options no longer fires when running with click >= 8.3. As a result, upload-coverage (and the individual commands) ignore auto-loadable parameters such as the repo slug, even though --auto-load-params-from is passed and the CI env vars are present. With --slug unset, the upload fails with:

ValueError: The provided slug is invalid
  File ".../codecov_cli/helpers/encoder.py", line 10, in encode_slug
    raise ValueError("The provided slug is invalid")

Reproduction

In a GitHub Actions-like environment (GITHUB_ACTIONS=true, GITHUB_REPOSITORY=owner/repo set) run without --slug:

codecov --auto-load-params-from githubactions upload-coverage \
  --network-root-folder . --flag myflag --disable-search \
  --commit-sha <sha> --token <token> -f coverage.txt

The --dry-run debug log shows "slug": null even though GITHUB_REPOSITORY is set and readable.

Expected vs actual

• Expected: slug is auto-loaded from the GitHub Actions adapter (GITHUB_REPOSITORY), as it was with click 8.2.x.
• Actual: slug stays None; encode_slug(None) raises "The provided slug is invalid".

Because super().get_default() now returns UNSET (truthy, not None), the method returns early and the CI-adapter / versioning-system fallback below becomes dead code.

Confirmed by introspection under click 8.3.1:

• adapter.get_fallback_value(FallbackFieldEnum.slug) → 'owner/repo' (adapter works)
• slug_option.get_default(ctx) → Sentinel.UNSET (fallback skipped)

Any CodecovOption field with no explicit default that relies on the fallback is affected (slug, git-service, etc.). --commit-sha masks the issue when passed explicitly.

Suggested fix

Treat click's UNSET sentinel as "no value" in the override, e.g.:

try:
    from click.core import UNSET  # click >= 8.3
except ImportError:
    UNSET = None

def get_default(self, ctx, call=True):
    res = super().get_default(ctx, call=call)
    if res is not None and res is not UNSET:
        return res
    ...
    return None   # normalize UNSET -> None on the fallback miss path too

Start in codecov_cli/fallbacks.py at CodecovOption.get_default and inspect how click 8.3.1's UNSET is handled. Reproduce with the provided GitHub Actions-like environment and upload-coverage command, then verify that an omitted --slug is loaded from GITHUB_REPOSITORY instead of producing null and the invalid-slug error.