`is_mutable_expr` causes false negatives in B039, RUF008, RUF012, and RUF024

[dscorbett]

Summary

The same issue as in #20004 affects mutable-contextvar-default (B039), mutable-dataclass-default (RUF008), mutable-class-default (RUF012), and mutable-fromkeys-value (RUF024). The extra cases in is_guaranteed_mutable_expr should be merged into is_mutable_expr. Examples:

from contextvars import ContextVar
ContextVar("cv", default=(x for x in "cv"))

from dataclasses import dataclass
@dataclass
class D:
    x: list[int] | object = ([],)

class C:
    x: list[object] = (x := [])
    y = (y := [])

dict.fromkeys(["A", "B"], ([],))

Version

ruff 0.14.0 (beea8cd 2025-10-07)