Closes gh-153534.

This broadens _asyncio's internal call_soon() helper to use the private
_call_soon() fast path for safe subclasses of stdlib
asyncio.base_events.BaseEventLoop, not just exact built-in loop types.

The fast path now applies only when the loop:

• is a stdlib BaseEventLoop subclass
• does not define call_soon on the subclass type
• does not shadow call_soon on the instance

Otherwise _asyncio falls back to the public call_soon() path.

This preserves behavior for custom loop behavior while expanding the
optimization to the common subclass-without-override case.

Notes

• Implementation note:
  • the guard does not cache BaseEventLoop during _asyncio init because
    _asyncio is imported while asyncio.base_events is still initializing
    and that creates an import cycle