gh-150361: Add test for _FlowControlMixin pause_writing exception handling

[azibom]

Summary

Add a unit test for _FlowControlMixin._maybe_pause_protocol() to cover the
exception handling path when protocol.pause_writing() raises an exception.

Motivation

The _maybe_pause_protocol() method contains an exception handling branch:

except BaseException as exc:
    self._loop.call_exception_handler({
        'message': 'protocol.pause_writing() failed',
        'exception': exc,
        'transport': self,
        'protocol': self._protocol,
    })

This path is currently not explicitly covered by tests.

Changes

This PR adds a test that:

• Triggers _maybe_pause_protocol() by exceeding the high-water mark
• Forces protocol.pause_writing() to raise RuntimeError
• Verifies loop.call_exception_handler() is called
• Checks that the correct error message is passed
• Ensures the original exception is forwarded correctly
• Confirms _protocol_paused remains True

• Issue: Add tests for asyncio _FlowControlMixin exception handling in pause_writing #150361