Support falsey classes for bool_value

Originally reported by: **Claudiu Popa (BitBucket: [PCManticore](http://bitbucket.org/PCManticore), GitHub: @PCManticore)**

---

`Class.bool_value` returns true for a class whose metaclass defines `__len__` to return 0 — i.e. astroid ignores the metaclass-level falseyness.

```python
from astroid import parse
cls = parse('''
class Meta(type):
def __len__(self):
return 0

class C(metaclass=Meta):
pass
''')['C']
print(cls.bool_value())
```

---

### Confirmed still reproducing on astroid 4.2.0b3 (2026-05-25)

Both the `bool_value()` API and the inference of `bool(C)` ignore the metaclass `__len__`:

```python
import astroid

ast = astroid.parse('''
class Meta(type):
def __len__(self):
return 0

class C(metaclass=Meta):
pass
''')
print(ast['C'].bool_value())
# astroid 4.2.0b3 -> True ❌ (real Python: False)

node = astroid.extract_node('''
class Meta(type):
def __len__(cls):
return 0
class C(metaclass=Meta):
pass
bool(C) #@
''')
print(node.inferred())
# astroid 4.2.0b3 -> [] ❌
# real Python -> False
```

Both `ClassDef.bool_value` and the builtin-`bool()` inference path need to consult the metaclass's `__bool__`/`__len__` (in MRO order) before falling back to the default truthy-class assumption.

---
- Bitbucket: https://bitbucket.org/logilab/astroid/issue/183

Start by tracing ClassDef.bool_value and the builtin-bool inference path, using the metaclass example in the issue as the reproducer. Check how metaclass __bool__ and __len__ are resolved in MRO order; done means both bool_value() and inferred bool(C) match Python and return False for the example.

Written by the indexing model from the issue text.