Fix RecursionError when assigning a benedict into its own nested descendant (#592)

[dualfroz]

Problem

#592

Assigning a benedict into a nested descendant of itself works once, but a
second such assignment crashes with RecursionError (and, per the issue,
can crash a Jupyter kernel):

from benedict import benedict

test = benedict(keyattr_enabled=True, keyattr_dynamic=True)
test.a.b.c = test        # works
test.a.b.d.e = test      # RecursionError: maximum recursion depth exceeded

Root cause

BaseDict._get_dict_or_value() in benedict/dicts/base/base_dict.py (lines
25-34 on main) recursively unwraps nested benedict values with no cycle
detection, and it is called from every __init__/__setitem__/update/
setdefault. After the first assignment, test contains a value that
(transitively) is test itself. Any later operation that needs to unwrap
test again (eg. navigating test.a to perform the second assignment)
recurses into that self-reference and never terminates.

The actual recursion is not a simple self-call of _get_dict_or_value: it
alternates between _get_dict_or_value, _cast/__init__ (which wrap the
same underlying raw dict into a fresh benedict object on every access) and
__setitem__ -> check_keys/traverse (benedict/core/traverse.py, which
has no cycle protection of its own either). All of these frames belong to
one continuous call stack, so a cycle guard confined to
_get_dict_or_value still sees the re-entry and can stop the whole chain
before the uncontrolled recursion is ever reached elsewhere.

Fix

benedict/dicts/base/base_dict.py: _get_dict_or_value now tracks, in a
class-level _unwrapping_ids set, the id() of every mapping it is
currently in the middle of unwrapping (added on entry, removed via
finally on exit -- so it only ever reflects containers that are live on
the current call stack, not a permanent "already seen" memo, which would
incorrectly flag legitimate shared/aliased references as cycles). If a
mapping is encountered while it is already being unwrapped higher up the
stack, the structure is self-referential: instead of recursing again (which
would eventually blow the stack, potentially inside unrelated,
unprotected code such as check_keys/traverse), a ValueError is raised
immediately, unwinding the whole unwrap/cast chain in a controlled way.

Normal (non-cyclic) nested-dict unwrapping is unaffected: the guard is only
ever populated with ids of mappings still being processed, and a proper
(acyclic) tree can never revisit a node that is still being visited, so the
new code paths are simply never exercised for legitimate data.

Scope note: a complete "silently supported" self-reference (making the
second assignment succeed rather than raise) is not achievable without
also touching benedict/core/traverse.py's _traverse_dict/
_traverse_collection (used, via check_keys, on every __setitem__, and
also by several other public APIs such as merge/flatten/keypaths),
since that traversal is independently unprotected against cycles. That was
considered out of scope for a minimal, low-risk fix, so this change turns
the uncontrolled RecursionError into a clear, immediate ValueError
instead.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.39%. Comparing base (ba7e146) to head (545cbf4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #597   +/-   ##
=======================================
  Coverage   98.38%   98.39%           
=======================================
  Files          64       64           
  Lines        2418     2425    +7     
=======================================
+ Hits         2379     2386    +7     
  Misses         39       39           

Flag	Coverage Δ
unittests	98.39% <100.00%> (+<0.01%)	⬆️

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