False negative: potentially unbound instance attribute doesn't trigger `possibly-undefined` error

Bug Report

When enabling the possibly-undefined error code, a potentially undefined instance attribute doesn't trigger the error, while a potentially unbound variable does.

To Reproduce

def test_possibly_undefined(flag: bool):
    if flag:
        a = 1
    print(a)  # error: Name "a" may be undefined  [possibly-undefined]


class Foo:
    def test_possibly_undefined(self, flag: bool):
        if flag:
            self.a = 1
        print(self.a)  # false negative, should also be a possibly-undefined error


try:
    test_possibly_undefined(False)
except Exception as e:
    print(f"{type(e).__name__}: {e}")  # UnboundLocalError: cannot access local variable 'a' where it is not associated with a value

f = Foo()
try:
    f.test_possibly_undefined(False)
except Exception as e:
    print(f"{type(e).__name__}: {e}")  # AttributeError: 'Foo' object has no attribute 'a'

Also available as mypy Playground, but cannot enable enable_error_code = possibly-undefined there.

Expected Behavior

A possible-undefined error should be reported for line 11 as well, e.g.:

test.py:4: error: Name "a" may be undefined  [possibly-undefined]
test.py:11: error: Attribute "a" may be undefined  [possibly-undefined]
Found 2 errors in 1 file (checked 1 source file)

Actual Behavior

No error reported for line 11.

test.py:4: error: Name "a" may be undefined  [possibly-undefined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.11.2
• Mypy command-line flags: --enable-error-code=possibly-undefined
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.12.6

Beginne damit, das Beispiel mit mypy 1.11.2 und dem possibly-undefined-Fehlercode zu reproduzieren, und finde dann die Checker-Logik und Regressionstests für möglicherweise nicht definierte lokale Variablen. Vergleiche die Behandlung lokaler Namen mit der von Instanzattributen und füge Abdeckung für das Foo-Beispiel hinzu; fertig ist die Änderung, wenn mypy den erwarteten Fehler Attribute "a" may be undefined meldet.