chore(digitalocean): enable untyped function checks

[Chan-Developer]

Proposed Commit Message

chore(digitalocean): enable untyped function checks

Type the complete DigitalOcean metadata stored by the datasource and
validate that it is available before reading DNS data for network
configuration. Remove the module from the mypy override list and add a
regression test for the incomplete metadata path.

Refs GH-5445

Additional Context

The normal datasource path is unchanged. If interface metadata exists but the complete metadata document has not been loaded, network_config now raises the datasource existing metadata error instead of failing while indexing None.

Test Steps

• python -m pytest -q tests/unittests/sources/test_digitalocean.py (14 passed)
• python -m mypy cloudinit/sources/DataSourceDigitalOcean.py
• Ruff, Pylint, Black, and isort on the changed Python files
• git diff --check

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)