Fix radius and diameter for digraphs with non-comparable vertices

[dcoudert]

Fixes #35300.

📚 Description

We specify an ordering of the vertices in the call to eccentricity. This way we fix both radius and diameter.

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: bab1966

[codecov-commenter]

Codecov Report

Patch coverage: 66.66% and project coverage change: -0.02 ⚠️

Comparison is base (c00e6c2) 88.62% compared to head (bab1966) 88.60%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35310      +/-   ##
===========================================
- Coverage    88.62%   88.60%   -0.02%     
===========================================
  Files         2148     2148              
  Lines       398855   398855              
===========================================
- Hits        353480   353414      -66     
- Misses       45375    45441      +66     

Impacted Files	Coverage Δ
src/sage/graphs/digraph.py	92.58% <66.66%> (-0.13%)	⬇️

... and 28 files with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[fchapoton]

ok