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/mysql/server.py

@@ -11,6 +11,7 @@ import base64
 import itertools
 import os
 import struct
+from typing import cast
 
 import instance_seed as _seed
 from syslog_bridge import syslog_line, write_syslog_file, forward_syslog
@@ -74,6 +75,9 @@ def _log(event_type: str, severity: int = 6, **kwargs) -> None:
 
 
 class MySQLProtocol(asyncio.Protocol):
+    _transport: asyncio.Transport | None = None
+    _peer: tuple[str, int] | None = None
+
     def __init__(self):
         self._transport = None
         self._peer = None
@@ -84,15 +88,16 @@ class MySQLProtocol(asyncio.Protocol):
         # same decky never present identical auth challenges.
         self._salt = _seed.fresh_bytes(20)
 
-    def connection_made(self, transport):
-        self._transport = transport
-        self._peer = transport.get_extra_info("peername", ("?", 0))
+    def connection_made(self, transport: asyncio.BaseTransport) -> None:
+        self._transport = cast(asyncio.Transport, transport)
+        self._peer = cast(tuple[str, int], self._transport.get_extra_info("peername", ("?", 0)))
         _log("connect", src=self._peer[0], src_port=self._peer[1],
              connection_id=self._conn_id)
-        transport.write(_make_packet(_build_greeting(self._conn_id, self._salt), seq=0))
+        self._transport.write(_make_packet(_build_greeting(self._conn_id, self._salt), seq=0))
         self._greeted = True
 
-    def data_received(self, data):
+    def data_received(self, data: bytes) -> None:
+        assert self._transport is not None
         self._buf += data
         # MySQL packets: 3-byte length + 1-byte seq + payload
         while len(self._buf) >= 4:
@@ -107,7 +112,8 @@ class MySQLProtocol(asyncio.Protocol):
             self._buf = self._buf[4 + length:]
             self._handle_packet(payload)
 
-    def _handle_packet(self, payload: bytes):
+    def _handle_packet(self, payload: bytes) -> None:
+        assert self._peer is not None
         if not payload:
             return
         # Login packet: capability flags (4), max_packet (4), charset (1),