[flake8-simplify] Respect side effects in lambda defaults (SIM401) - #28000

Summary

Fix SIM401 false positives when a lambda fallback has side-effectful parameter defaults:

def get_callback(callbacks: dict, key, initialize):
    if key in callbacks:
        callback = callbacks[key]
    else:
        callback = lambda value=initialize(): value
    return callback

Replacing the conditional with callback = callbacks.get(key, lambda value=initialize(): value) calls initialize() even when the key exists. The conditional only calls it when the key is missing.

any_over_expr now visits lambda parameter defaults before the body. This extracts the helper change from #27634.

The existing SIM401 snapshot fixture now covers positional-only, positional-or-keyword, and keyword-only defaults, including a nested call and a literal-default case that still gets a diagnostic.