CI: stand up Gitea Actions (TST-03) + fix the defects it caught (#123)

Make the authored CI pipeline execute and go green on the co-located runner (portable + java), fix the five real latent defects the first real run surfaced (codegen-check null, stale rust proto, orphan-terminator Linux path, stale java worker codegen, py3.12 event loop), provision pwsh + Gradle for the self-hosted act image, and disable the Windows jobs (act host-mode broken). TST-03 -> Done.

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