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.

templates/mysql/server.py

@@ -67,6 +67,10 @@ class MySQLProtocol(asyncio.Protocol):
         # MySQL packets: 3-byte length + 1-byte seq + payload
         while len(self._buf) >= 4:
             length = struct.unpack("<I", self._buf[:3] + b"\x00")[0]
+            if length > 1024 * 1024:
+                self._transport.close()
+                self._buf = b""
+                return
             if len(self._buf) < 4 + length:
                 break
             payload = self._buf[4:4 + length]