fix(protocols): guard against zero/malformed length fields in binary protocol parsers

MongoDB had the same infinite-loop bug as MSSQL (msg_len=0 → buffer never
shrinks in while loop). Postgres, MySQL, and MQTT had related length-field
issues (stuck state, resource exhaustion, overlong remaining-length).

Also fixes an existing MongoDB _op_reply struct.pack format bug (extra 'q'
specifier caused struct.error on any OP_QUERY response).

Adds 53 regression + protocol boundary tests across MSSQL, MongoDB,
Postgres, MySQL, and MQTT, including a _run_with_timeout threading harness
to catch infinite loops and @pytest.mark.fuzz hypothesis tests for each.

tests/service_testing/conftest.py

@@ -0,0 +1,47 @@
+"""
+Shared helpers for binary-protocol service tests.
+"""
+
+import os
+import threading
+from types import ModuleType
+from unittest.mock import MagicMock
+
+import pytest
+from hypothesis import HealthCheck
+
+
+_FUZZ_SETTINGS = dict(
+    max_examples=int(os.environ.get("HYPOTHESIS_MAX_EXAMPLES", "200")),
+    deadline=2000,
+    suppress_health_check=[HealthCheck.function_scoped_fixture],
+)
+
+
+def make_fake_decnet_logging() -> ModuleType:
+    mod = ModuleType("decnet_logging")
+    mod.syslog_line = MagicMock(return_value="")
+    mod.write_syslog_file = MagicMock()
+    mod.forward_syslog = MagicMock()
+    mod.SEVERITY_WARNING = 4
+    mod.SEVERITY_INFO = 6
+    return mod
+
+
+def run_with_timeout(fn, *args, timeout: float = 2.0) -> None:
+    """Run fn(*args) in a daemon thread. pytest.fail if it doesn't return in time."""
+    exc_box: list[BaseException] = []
+
+    def _target():
+        try:
+            fn(*args)
+        except Exception as e:
+            exc_box.append(e)
+
+    t = threading.Thread(target=_target, daemon=True)
+    t.start()
+    t.join(timeout)
+    if t.is_alive():
+        pytest.fail(f"data_received hung for >{timeout}s — likely infinite loop")
+    if exc_box:
+        raise exc_box[0]