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/rdp/server.py

@@ -331,6 +331,7 @@ async def _upgrade_to_tls_and_capture(
     # into a StreamReader/StreamWriter pair the rest of the handler can use.
     new_reader = asyncio.StreamReader(loop=loop)
     new_protocol = asyncio.StreamReaderProtocol(new_reader, loop=loop)
+    assert new_transport is not None
     new_transport.set_protocol(new_protocol)
     new_protocol.connection_made(new_transport)
     new_writer = asyncio.StreamWriter(new_transport, new_protocol, new_reader, loop)