fix: clarify why @validator decorator cannot be used in sendfile example

[digital-wizard48]

Summary

• The sendfile.py example had @validator commented out with only # breaks sendfile as explanation
• The root cause is that wsgiref.validate.validator wraps the return value of the app in another iterable, so the isinstance(respiter, environ['wsgi.file_wrapper']) check in the sync worker fails
• When that check fails, gunicorn falls back to iterating the response normally instead of using the sendfile syscall optimization
• This PR improves the comment to clearly explain why @validator cannot be used with the sendfile example

Fixes #2279

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This PR was auto-generated by Gittensor bot using Claude AI to fix a reported issue.

[benoitc]

why did you remove the @validator decoration though?