fix(types): P3 — annotate transport in all template protocol servers; 0 errors in templates/

- asyncio.Protocol (TCP): _transport: asyncio.Transport | None = None + cast() in
  connection_made; assert guards in every method that directly accesses the field.
  Files: pop3, smtp, mqtt, postgres, mssql, mongodb, imap, ldap, redis, mysql, sip, vnc.
- asyncio.DatagramProtocol (UDP): _transport: asyncio.DatagramTransport | None = None.
  Files: snmp, tftp, SIPUDPProtocol.
- RDP: assert new_transport is not None after start_tls() to narrow Transport | None.
- FTP (Twisted): assert self.transport is not None + targeted type: ignore for imprecise
  Twisted stubs (misc/override/arg-type/attr-defined), IReactorTCP cast for listenTCP.
- conpot: proc.stdout is None guard before iteration.
- Bonus fixes surfaced by annotation:
  - smtp: get_payload(decode=True) bytes narrowing (arg-type on sha256)
  - postgres: rename shadowed `msg` param to `err_msg` in _handle_startup
  - mongodb: base64.binascii.Error → import binascii; binascii.Error
  - imap: result: list[int] = [] (var-annotated)

decnet/templates/snmp/server.py

@@ -9,6 +9,7 @@ Logs all requests as JSON.
 import asyncio
 import os
 import struct
+from typing import cast
 from syslog_bridge import (
     encode_secret,
     forward_syslog,
@@ -225,11 +226,13 @@ def _build_response(version: int, community: str, request_id: int, oids: list) -
 
 
 class SNMPProtocol(asyncio.DatagramProtocol):
+    _transport: asyncio.DatagramTransport | None = None
+
     def __init__(self):
         self._transport = None
 
-    def connection_made(self, transport):
-        self._transport = transport
+    def connection_made(self, transport: asyncio.BaseTransport) -> None:
+        self._transport = cast(asyncio.DatagramTransport, transport)
 
     def datagram_received(self, data, addr):
         try:
@@ -244,7 +247,8 @@ class SNMPProtocol(asyncio.DatagramProtocol):
                  principal=None, secret_kind="snmp_community",
                  **encode_secret(community))
             response = _build_response(version, community, request_id, oids)
-            self._transport.sendto(response, addr)
+            if self._transport is not None:
+                self._transport.sendto(response, addr)
         except Exception as e:
             _log("parse_error", severity=4, src=addr[0], error=str(e), data=data[:64].hex())