test: reproducer for pyzmq Context.__del__ ioloop wedge

Add tests/pytests/functional/transport/zeromq/test_context_finalizer_wedge.py
documenting the pyzmq >= 24 Context.__del__ -> destroy() -> socket.close()
behavior that freezes a salt-minion's asyncio ioloop when a leaked
zmq.Context with a queued undeliverable send is finalized inside a
callback.

Four tests:

- test_pyzmq_context_del_hangs_baseline (xfail(strict=True)): bare pyzmq
  primitive wedges > 4s on shipped pyzmq 27.1.0.
- test_ioloop_freezes_when_leaked_context_finalized_in_callback
  (xfail(strict=True)): production failure shape -- leaked Context
  finalized inside asyncio callback freezes the loop; canary tick
  counter measures the freeze.
- test_explicit_destroy_linger_zero_prevents_ioloop_freeze: documents
  the fix pattern (ctx.destroy(linger=0) before drop).
- test_salt_zeromq_transport_defines_no_dunder_del: static regression
  guard that no __del__ is (re)introduced on salt.transport.zeromq
  classes.

The wedge test bodies run under a daemon-thread bound so a wedged
zmq_ctx_term() cannot hang the test suite; pytest still reports
XFAIL/PASS cleanly.

Verified on salt onedir (pyzmq 27.1.0, python 3.14.6):
  2 passed, 2 xfailed, 100 warnings in 16.05s

Author: dwoz