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.
