_looks_like_connect_timeout and _looks_like_pool_timeout carried two
copies of the same 15-line DFS skeleton (seen-set, stack, __cause__/
__context__ descent) differing only in the one-line match predicate —
follow-up to the #98094 review.
Extract _iter_exception_graph() and collapse both classifiers onto it.
Behavior is byte-identical (subprocess parity vs origin/main on real PTB
error fixtures: 6/6 identical), and the two classifiers gain direct unit
tests for the first time, including the cycle/diamond chain shapes the
inline copies had no coverage for.
6681f9ebc3