gh-152099: Raise SendfileNotAvailableError for fallback-only transports

[zainnadeem786]

Summary

This PR makes BaseEventLoop.sendfile() consistently raise asyncio.SendfileNotAvailableError when native sendfile cannot be used for fallback-capable transports (such as SSL/TLS) and fallback=False.

Currently, native sendfile failures on _SendfileMode.TRY_NATIVE transports already raise SendfileNotAvailableError, while _SendfileMode.FALLBACK transports raise a generic RuntimeError. This results in inconsistent exception behavior for the same practical condition: native sendfile is unavailable and fallback has been disabled.

The change updates only the fallback-only transport path. Unsupported transports and closing transports continue to raise RuntimeError unchanged.

Changes

• Raise SendfileNotAvailableError instead of RuntimeError for _SendfileMode.FALLBACK when fallback=False.
• Update the corresponding asyncio sendfile test to expect the public exception type.
• Keep all other code paths unchanged.

Validation

Executed:

PCbuild\amd64\python_d.exe -m test test_asyncio -m test_sendfile_no_fallback_for_fallback_transport
PCbuild\amd64\python_d.exe -m test test_asyncio.test_sendfile
git diff --check

All focused tests passed.

Compatibility

SendfileNotAvailableError subclasses RuntimeError, so existing code catching RuntimeError continues to work while allowing applications to reliably detect native sendfile unavailability.

Closes gh-152099.