gh-154817: Fix OrderedDict.pop() crash with inconsistent equality

[BHUVANSH855]

Summary

Fix a crash in collections.OrderedDict.pop() when a key's __eq__
implementation returns inconsistent results across repeated comparisons.

If the initial lookup succeeds but the subsequent removal lookup fails,
and no default value is supplied, OrderedDict.pop() could dereference
a NULL default value and crash the interpreter.

This change raises KeyError instead of dereferencing the NULL default
value and adds a C implementation regression test covering this scenario.

The separate OrderedDict container desynchronization issue is tracked
in gh-154835.

Changes

• Fix the missing-key path in OrderedDict.pop() when no default value is provided.
• Raise KeyError instead of dereferencing a NULL default value.
• Add a C implementation regression test for a key with stateful equality.

Testing

./python -m test test_ordered_dict
./python -m test test_collections

Closes gh-154817

[StanFromIreland]

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