fix: stop hanging on cyclic exception __cause__ chains

[MohammedAnasNathani]

Summary

Fixes #658.

retry_if_exception_cause_type walked __cause__ until None. A cyclic cause chain never ends, so the predicate spins at 100% CPU and stop conditions never run (they are only checked after the predicate returns).

This is legal Python:

def boom():
    try:
        raise ValueError("inner")
    except ValueError as e:
        raise e from e  # self-referential __cause__

Retrying(
    retry=retry_if_exception_cause_type(KeyError),
    stop=stop_after_attempt(2),
    reraise=True,
)(boom)  # hung forever

Two-node cycles (a.__cause__ = b; b.__cause__ = a) hang the same way.

Fix

Track id(exc) while walking and stop when a cycle is detected — the same idea as the stdlib traceback module’s cycle-safe chain walk.

Tests

• Self-cause cycle (raise e from e)
• Two-node mutual cause cycle
• Existing cause-type tests still pass

Checklist

• Regression tests
• Local pytest -k exception_cause green

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Merge Queue Status

• ✅ Entered queue — 2026-07-29 10:39 UTC · Rule: default · triggered by rule autoqueue
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-07-29 10:39 UTC · at b1936f1cc172b17e3fcd45ed84416f7e4c191fc0 · squash

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• github-review-approved [🛡 GitHub branch protection]
  • fix: stop hanging on cyclic exception __cause__ chains #660