ci(tst-03): ensure a current event loop for sync python tests (py3.12)

The last portable failure: pytest-asyncio 1.x does not install a current event
loop for synchronous tests and clears it after each async test, so a later sync
test that builds a grpc.aio.Channel (create_channel) hit
"RuntimeError: There is no current event loop" on Python 3.12 depending on test
order (CI hit it; local order did not). Add an autouse conftest fixture that
ensures a usable current loop per test. Local: 127 passed, 1 skipped.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

clients/python/tests/conftest.py

@@ -0,0 +1,27 @@
+"""Shared pytest fixtures for the client test suite.
+
+pytest-asyncio 1.x no longer installs a current event loop for *synchronous*
+tests, and it clears the loop after each ``async`` test. Some synchronous tests
+build a ``grpc.aio.Channel`` (via ``create_channel``), which calls
+``asyncio.get_event_loop()`` and therefore raises
+``RuntimeError: There is no current event loop`` on Python 3.12 when a prior
+async test has left the policy with no current loop. Ensure every test starts
+with a usable current loop so channel construction in a sync test is
+order-independent.
+"""
+
+from __future__ import annotations
+
+import asyncio
+from collections.abc import Iterator
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _ensure_current_event_loop() -> Iterator[None]:
+    try:
+        asyncio.get_event_loop()
+    except RuntimeError:
+        asyncio.set_event_loop(asyncio.new_event_loop())
+    yield