test(issue-26): no test depends on a third-party MQTT broker

- tests/_local_mqtt.py: in-process MQTT 3.1.1 broker on 127.0.0.1 (what
  the repo uses: protocol level 4, clean session, QoS 0/1, retained).
- conftest: one session broker named by MQTT_BROKER/MQTT_PORT/MQTT_TLS;
  other redirecting/auth MQTT_* dropped and restored afterwards.
- Harnesses of the 22 broker-dependent tests (#18 acceptance1/2/3/5,
  #6 t1_1/2/7/8/14/15/16/20, #19 MQTT loop x2, #33 b3/r3/b7, #30
  s9/s10/r2) use it; the "broker unreachable" skips are gone (a local
  broker that does not deliver is a failure). acceptance1/5 and #19 also
  assert on the subscriber, runner and audit logs the real producers write.
- tests/test_local_mqtt.py: conformance with the real clients and the real
  publish_event -> job_subscriber path (C0), an AST guard against public
  broker literals and MQTT_* deleted without a replacement (C2), and the
  session's broker config (C3).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -1550,3 +1550,35 @@ def delegation_tree_provenance_guard(tmp_path_factory):
         "(these would shadow the real tree for any later first-time import):\n  "
         + "\n  ".join(polluted)
     )
+
+
+# ==============================================================================
+# Issue #26: no test reaches a third-party MQTT broker. One in-process broker
+# (tests/_local_mqtt.py) per session on 127.0.0.1; MQTT_BROKER/MQTT_PORT/
+# MQTT_TLS name it, and every other MQTT_* that would redirect or authenticate
+# a client is dropped. Subprocesses inherit it through os.environ; a harness
+# that builds its own env takes _local_mqtt.local_broker_env(). Tests that
+# deliberately name an unreachable broker (127.0.0.1:9, :65534) still win.
+# ==============================================================================
+import _local_mqtt  # noqa: E402
+
+_MQTT_DROPPED = ("MQTT_HOST", "MQTT_USERNAME", "MQTT_PASSWORD", "MQTT_CA_CERTS", "MQTT_CERTFILE", "MQTT_KEYFILE")
+
+
+@pytest.fixture(scope="session", autouse=True)
+def local_mqtt_broker():
+    broker = _local_mqtt.start_broker()
+    keys = tuple(broker.env()) + _MQTT_DROPPED
+    previous = {k: os.environ.get(k) for k in keys}
+    for k in _MQTT_DROPPED:
+        os.environ.pop(k, None)
+    os.environ.update(broker.env())
+    try:
+        yield broker
+    finally:
+        for k, v in previous.items():
+            if v is None:
+                os.environ.pop(k, None)
+            else:
+                os.environ[k] = v
+        _local_mqtt.stop_broker()