Don't resolve implicit __bool__ through __getattr__

[devteamaegis]

Summary

Fixes #4467.

Implicit dunder methods such as __bool__ are looked up on the type and bypass __getattr__ at runtime (CPython resolves them via _PyObject_LookupSpecial, which ignores both the instance dict and __getattr__/__getattribute__).

When evaluating truthiness in as_bool, Pyrefly looked up __bool__ with the __getattr__ fallback enabled. So a class whose __getattr__ returns a non-callable type — e.g. torch.nn.Module, whose __getattr__ is typed to return Tensor | Module — was treated as defining a non-callable __bool__, and using an instance in a boolean context was wrongly reported as not-callable:

class Tensor: ...
class M:
    def __getattr__(self, name: str) -> "Tensor | M": ...

def f(x: M | None) -> M:
    return x or M()
    # before: ERROR Expected `__bool__` to be a callable, got `M | Tensor` [not-callable]
    # after:  0 errors

The fix disables the __getattr__ fallback for the __bool__ lookup in as_bool (pyrefly/lib/alt/expr.rs). The separate check_dunder_bool_is_callable path already disables the fallback; this brings the truthiness-evaluation path in line. The subsequent __bool__ call is only reached when a real __bool__ is found directly, so classes that genuinely define __bool__ are unaffected.

Test Plan

• Added test_getattr_does_not_provide_dunder_bool in pyrefly/lib/test/attributes.rs. It fails before the change (expected 0 errors, but got 2) and passes after.
• cargo test -p pyrefly --lib: all type-checking tests pass (the only failures on my machine are two pre-existing lsp_interaction::diagnostic timeout tests that also fail on unmodified main, unrelated to this change).
• cargo clippy -p pyrefly --lib and cargo fmt --check are clean for the changed files.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D115946422. (Because this pull request was imported automatically, there will not be any future comments.)