chore: add type annotation to cloudinit.net.network_manager

[anujsingh-cse]

Proposed Commit Message

chore: add type annotation to cloudinit.net.network_manager

Removing cloudinit.net.network_manager from the mypy override list in
pyproject.toml surfaces two errors on the same line.

__init__ assigns the builtin str as the optionxform transform, the
documented configparser idiom for preserving option name case. mypy
cannot express assigning a class to a method, so it is ignored with a
comment explaining why, matching the comment added in
cloudinit/sources/helpers/vmware/imc/config_file.py.

Refs GH-5445

Additional Context

Refs GH-5445. Claimed in #5445 (comment).

Removing the module from the override list surfaces two errors:

network_manager.py:56: error: Cannot assign to a method  [method-assign]
network_manager.py:56: error: Incompatible types in assignment
                (expression has type "type[str]",
                 variable has type "Callable[[str], str]")  [assignment]

self.config.optionxform = str is the documented configparser pattern
for preserving option name case (NM requires it). mypy cannot express
assigning a class to a method, so the line is ignored — with a comment
documenting why, per the review feedback on #7098.

Test Steps

$ python -m mypy cloudinit/net/network_manager.py
(0 errors with the override removed)

$ pytest tests/unittests/net/test_network_manager.py -q
7 passed

$ ruff check / black --check / isort --check-only (target file)
All green

tox -e py3 and tox -e check_format to be confirmed by CI
(developed on Windows where the full tox suite cannot run).

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>)