blockbuster: allow os.sendfile from asyncio selector loop

[agners]

Proposed change

Serving aiohttp FileResponse triggers BlockBuster's os.sendfile guard because the actual syscall happens in asyncio/unix_events.py's _sock_sendfile_native_impl rather than the base_events.sendfile entry point that blockbuster whitelists. The socket is non-blocking and EAGAIN is handled by re-registering a writer, so the call is safe.

Extend the os.sendfile allowlist with the selector-loop caller as a local workaround. Fixed upstream by
cbornet/blockbuster#58 (issue cbornet/blockbuster#57); we can remove this work around once blockbuster is bumped to a version that includes the fix.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to the supervisor)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
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Checklist

• The code change is tested and works locally.
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• The code has been formatted using Ruff (ruff format supervisor tests)
• Tests have been added to verify that the new code works.

If API endpoints or add-on configuration are added/changed:

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• Client library updated (if necessary)