tools/ynl: Make YnlFamily closeable as a context manager

tools/ynl: Make YnlFamily closeable as a context manager

YnlFamily opens an AF_NETLINK socket in __init__ but has no way
to release it other than leaving it to the GC. YnlFamily holds a
self reference cycle through SpecFamily's self.family = self
in its super().__init__() call, so refcount GC cannot reclaim
it and the socket stays open until the cyclic GC runs.

If a test creates a guest netns, instantiates a YnlFamily inside
it via NetNSEnter(), performs some test case work via Ynl, and
then deletes the netns, then the 'ip netns del' only drops the
mount binding and cleanup_net in the kernel never runs, so any
subsequent test case assertions that objects got cleaned up would
fail given this only gets triggered later via cyclic GC run.

Add an explicit close() that closes the netlink socket and wire
up the __enter__/__exit__ so callers can scope the instance
deterministically via 'with YnlFamily(...) as ynl: ...'.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260413220809.604592-2-daniel@iogearbox.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>