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/test_issue19_subscriber_daemon_lifecycle.py

@@ -28,6 +28,8 @@ from pathlib import Path
 
 import pytest
 
+from _local_mqtt import local_broker_env
+
 FIXTURES = Path(__file__).parent / "fixtures" / "issue_5" / "phase1"
 DELEGATION_MODES = ("0", "1")
 
@@ -415,11 +417,12 @@ def test_sub_stop_trap_exits_143_before_the_first_background_child():
 # ---------------------------------------------------------------------------
 
 def _start_connected(mam_sandbox, log):
-    proc = _start_subscriber(mam_sandbox, log, {})
-    if not _wait_for(log, r"MQTT Monitor: subscribed to ", 45):
+    # Issue #26: the session's in-process broker is always up - not reaching it fails.
+    proc = _start_subscriber(mam_sandbox, log, local_broker_env())
+    if not _wait_for(log, r"MQTT Monitor: subscribed to mam/[0-9a-f]{12}/jobs/\+/events", 45):
         proc.kill()
         proc.wait(timeout=10)
-        pytest.skip("broker unreachable from this host: " + log.read_text()[-300:])
+        pytest.fail("the monitor daemon never subscribed on the local broker: " + log.read_text()[-300:])
     return proc