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/smtp/server.py

@@ -30,6 +30,7 @@ from datetime import datetime, timezone
 from email import message_from_bytes
 from email.header import decode_header, make_header
 from email.message import Message
+from typing import cast
 
 import instance_seed as _seed
 from syslog_bridge import (
@@ -150,7 +151,8 @@ def _summarize_message(body: bytes, msg_id: str) -> dict:
         if not filename and "attachment" not in disposition:
             continue
         try:
-            payload = part.get_payload(decode=True) or b""
+            _raw = part.get_payload(decode=True) or b""
+            payload: bytes = _raw if isinstance(_raw, bytes) else b""
         except Exception:
             payload = b""
         attachments.append({
@@ -207,6 +209,9 @@ def _decode_auth_plain(blob: str) -> tuple[str, str]:
 
 
 class SMTPProtocol(asyncio.Protocol):
+    _transport: asyncio.Transport | None = None
+    _peer: tuple[str, int]
+
     def __init__(self):
         self._transport = None
         self._peer      = ("?", 0)
@@ -228,11 +233,11 @@ class SMTPProtocol(asyncio.Protocol):
 
     # ── asyncio.Protocol ──────────────────────────────────────────────────────
 
-    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 = self._transport.get_extra_info("peername", ("?", 0))
         _log("connect", src=self._peer[0], src_port=self._peer[1])
-        transport.write(f"{_SMTP_BANNER}\r\n".encode())
+        self._transport.write(f"{_SMTP_BANNER}\r\n".encode())
 
     def data_received(self, data):
         self._buf += data
@@ -247,6 +252,7 @@ class SMTPProtocol(asyncio.Protocol):
     # ── Command dispatch ──────────────────────────────────────────────────────
 
     def _handle_line(self, line: str) -> None:
+        assert self._transport is not None
         # ── DATA body accumulation ────────────────────────────────────────────
         if self._in_data:
             if line == ".":
@@ -444,6 +450,7 @@ class SMTPProtocol(asyncio.Protocol):
     # ── AUTH helpers ──────────────────────────────────────────────────────────
 
     def _handle_auth(self, args: str) -> None:
+        assert self._transport is not None
         parts    = args.split(None, 1)
         mech     = parts[0].upper() if parts else ""
         initial  = parts[1] if len(parts) > 1 else ""
@@ -468,6 +475,7 @@ class SMTPProtocol(asyncio.Protocol):
             self._transport.write(b"504 5.5.4 Unrecognized authentication mechanism\r\n")
 
     def _finish_auth(self, username: str, password: str) -> None:
+        assert self._transport is not None
         _log("auth_attempt", src=self._peer[0],
              username=username, principal=username,
              severity=SEVERITY_WARNING, **encode_secret(password))