chore(nocloud): enable untyped function checks

[dwaradwara]

Proposed Commit Message

chore(nocloud): enable untyped function checks

Add typing needed for strict function-body checking and remove the
NoCloud check_untyped_defs exemption.

Refs GH-5445

Additional Context

Enable strict function-body type checking for DataSourceNoCloud.

This adds the type information required by mypy, removes
DataSourceNoCloud from the check_untyped_defs exemption, and annotates
the shared DataSource _platform_type attribute to match its existing
string-or-None behavior.

No intended runtime behavior changes.

Test Steps

tox -e mypy -- cloudinit/sources/DataSourceNoCloud.py
tox -e py3 -- tests/unittests/sources/test_nocloud.py
tox -e ruff -- cloudinit/sources/DataSourceNoCloud.py cloudinit/sources/__init__.py
tox -e black -- cloudinit/sources/DataSourceNoCloud.py cloudinit/sources/__init__.py
tox -e isort -- cloudinit/sources/DataSourceNoCloud.py cloudinit/sources/__init__.py
git diff --check

NoCloud unit tests: 19 passed.

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits.