Conditional annotations are always included in `STRING` annotations

Bug report

Bug description:

If an annotation is excluded via a conditional block it is correctly excluded in VALUE and FORWARDREF annotations but is incorrectly included under STRING annotations.

from annotationlib import get_annotations, Format

class Example:
    a: int
    if False:
        b: str

print(get_annotations(Example))
print(get_annotations(Example, format=Format.STRING))

{'a': <class 'int'>}
{'a': 'int', 'b': 'str'}

'b' should not be included in string annotations here.

CPython versions tested on:

3.14, 3.15, CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-156516

先使用 get_annotations 和 Format.STRING 重现该示例，然后检查 annotationlib 的实现及其相关测试。完成的标准是：STRING 结果中不包含 b 等条件注解，与 VALUE 和 FORWARDREF 的行为一致，并且该回归由通过的测试覆盖。