enhancement(typing): add type annotations to cloudinit.sources.helpers.netlink (#5445)

[AhmadBilalDSA]

Addresses part of #5445 by adding strict type annotations to cloudinit.sources.helpers.netlink to satisfy --disallow-untyped-defs.

Changes Made

• Added type hints to all public helper functions, methods, and return signatures in cloudinit/sources/helpers/netlink.py.
• Fully annotated internal callback signatures (should_continue_cb).
• Added # type: ignore[attr-defined] guards for platform-specific socket attributes (AF_NETLINK, NETLINK_ROUTE).
• Verified zero errors with mypy --disallow-untyped-defs.

Type of Change

• Code quality / Typing improvement

Addresses #5445

[blackboxsw]

Thank your for this submission and helping improve quality in cloud-init.

I think there are a couple of thngs to resolve to move forward with this PR.

1. Please avoid adding more # type: ignore comments to the code and actually resolve the typing issues in the file to progress toward proper typing coverage.
2. make tox -e mypy succeed
3. update docstr :returns: of create_bound_netlink_socket and get_netlink_msg_header to represent that actual instance type returned.

[AhmadBilalDSA]

Thank you for the review, @blackboxsw! I have addressed all feedback points in the latest commit:

1. Avoid # type: ignore: Replaced direct access to socket.AF_NETLINK and socket.NETLINK_ROUTE with getattr(socket, 'AF_NETLINK', 16) and getattr(socket, 'NETLINK_ROUTE', 0), resolving the typing issue cleanly without ignores. 2. Type checker: Verified that mypy cloudinit/sources/helpers/netlink.py passes cleanly (Success: no issues found in 1 source file). 3. Docstring updates: Updated :returns: for both create_bound_netlink_socket and get_netlink_msg_header to document the concrete instance types (socket.socket and NetlinkHeader)

[AhmadBilalDSA]

All four feedback items have been addressed in cloudinit/sources/helpers/netlink.py:

1. Type safety without # type: ignore or getattr fallbacks:
   Replaced getattr(socket, "AF_NETLINK", 16) and getattr(socket, "NETLINK_ROUTE", 0) with standard socket.AF_NETLINK and socket.NETLINK_ROUTE inside a sys.platform == "linux" guard. It raises NetlinkCreateSocketError on non-Linux platforms rather than relying on fallback magic numbers. MyPy cleanly narrows sys.platform via typeshed without requiring ignores.

2. Docstrings:
   Verified and retained explicit return types (socket.socket at line 65 and NetlinkHeader namedtuple at line 92).

3. Elimination of # type: ignore:
   Removed # type: ignore[unreachable] and the dead if data is None branch in read_netlink_socket. netlink_socket.recv() returns bytes per standard library type stubs, so it directly returns netlink_socket.recv(MAX_SIZE). The file now contains zero # type: ignore pragmas.

4. Verification:

   • Static typing: mypy passes cleanly under both default (Windows) and --platform linux.
   • Formatting/linting: ruff check and black pass with zero violations.