Handle empty HUGGING_FACE_TOKEN gracefully

[MaxGhenis]

Summary

• Treat empty string token same as None (fixes CI with missing secrets)
• Check os.isatty(0) before prompting - return None in non-interactive environments instead of hanging on getpass
• Update return type to str | None

Problem

When Dependabot opens a PR in repos using policyengine-core, it doesn't have access to repository secrets. The HUGGING_FACE_TOKEN secret gets passed as an empty string "", which causes:

1. get_or_prompt_hf_token() checks if token is None: - but "" is not None
2. Returns empty string ""
3. hf_hub_download tries to use Bearer header (empty token)
4. httpx fails with Illegal header value b'Bearer '

Solution

# Before
if token is None:
    token = getpass(...)

# After  
if not token:  # Catches both None and ""
    if os.isatty(0):  # Only prompt if interactive
        token = getpass(...)
    else:
        return None  # CI can handle None gracefully

Related

• Companion PR in policyengine-uk: Handle missing HUGGING_FACE_TOKEN gracefully in CI policyengine-uk#1480
• This is the proper fix at the source; the UK PR is a workaround

Test plan

• CI passes
• Manually verify empty token returns None in non-interactive mode
• Verify interactive prompt still works when running locally

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.41%. Comparing base (58c2fca) to head (1e709ba).
⚠️ Report is 29 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
+ Coverage   81.14%   82.41%   +1.27%     
==========================================
  Files         197      201       +4     
  Lines       10228    10517     +289     
  Branches     1057     1064       +7     
==========================================
+ Hits         8299     8668     +369     
+ Misses       1640     1574      -66     
+ Partials      289      275      -14     

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