Fix TypeError when a parenthesized lambda follows a keyword

[abhishekacharya200]

Summary

Fixes #1061.

Lambda._safe_to_use_with_word_operator called zero-argument super(). Because @add_slots rebuilds the class, the __class__ cell used by super() still points at the original class, so the call fails with TypeError: super(type, obj): obj must be an instance or subtype of type. This happens whenever a parenthesized lambda sits right after a keyword with no space, e.g.:

import libcst
libcst.parse_module("() if 0 else(lambda:())")
libcst.parse_module("x = not(lambda: 0)")
libcst.parse_module("x = a or(lambda: 0)")

The fix uses super(Lambda, self), which is what the other slotted nodes in expression.py (and Lambda._validate) already do.

Test Plan

Added two cases to libcst/_nodes/tests/test_ifexp.py:

• a valid foo if bar else(lambda:baz) node (round-trips through parse_expression)
• an invalid unparenthesized lambda after else with no space, which now raises the expected CSTValidationError instead of a TypeError

python -m unittest libcst._nodes.tests.test_ifexp libcst._nodes.tests.test_lambda

Without the fix the module errors with the TypeError above; with it, all tests pass. The full python -m libcst.tests run shows no new failures compared to main. flake8 and ufmt check pass on the changed files.

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