test: add unit tests for _is_cancel_scope_runtime_error helper

The existing regression test only exercises the False return path
(connection error is NOT a cancel scope error). That left lines 423,
425-426 uncovered — the True return paths when a cancel scope
RuntimeError IS detected (direct, via __cause__ chain, via
__context__ chain, or inside a BaseExceptionGroup).

Add 10 focused unit tests covering all True paths plus edge cases
(None, non-RuntimeError, non-matching groups, circular references).

Author: VRTXOmega