Exempt nested blueprints when the parent is CSRF-exempt

[gyanu2507]

_is_exempt only compared the current blueprint object, so a nested child like api.meta stayed protected after csrf.exempt(api).

Walk dotted names so an exempt parent covers its children.

• fixes Add support for exemption on nested blueprints in CSRFProtect #697

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in docs/changes.rst summarizing the change and linking to the issue. Add .. versionchanged:: entries in any relevant code docs.

[davidism]

https://palletsprojects.com/contributing/llm-ai