fix: clear all mypy errors

Annotate untyped containers and Optional attributes, type the stdlib-shaped
metric dicts in the client plugins, and add types-PyYAML/types-psutil to the
dev extras plus a mypy override for nio/mattermostdriver (no stubs).

Behaviour fixes found on the way: enable_kernel_timestamps() now returns False
when SO_TIMESTAMP is unknown instead of raising TypeError, send_shutdown is
defined at module level so cleanup() cannot hit a NameError, and a failed
subprocess returncode falls back to UNKNOWN.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

hbd/server/udp.py

@@ -5,6 +5,7 @@ import socket
 import struct
 import time
 import zlib
+from typing import Optional
 import logging
 
 from platform import system as platform_system
@@ -25,6 +26,7 @@ _STALE_INTERVAL_MULTIPLIER = 3
 # Supported on Linux, FreeBSD, and macOS.  The constant is not exposed by
 # Python's socket module on all platforms
 platform = platform_system()
+_SO_TIMESTAMP: Optional[int]
 if platform == "Darwin":
     _SO_TIMESTAMP = 1024  # SO_TIMESTAMP on macOS (not in Python's socket module)
 elif platform == "Linux":
@@ -45,6 +47,8 @@ def enable_kernel_timestamps(sock) -> bool:
     Returns True if the kernel will supply receive timestamps, False otherwise
     (unsupported platform, older kernel, or insufficient permissions).
     """
+    if _SO_TIMESTAMP is None:
+        return False
     try:
         sock.setsockopt(socket.SOL_SOCKET, _SO_TIMESTAMP, 1)
         return True
@@ -430,7 +434,7 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         return
 
     cfg = ctx.get("config", {})
-    hbdcls = ctx.get("hbdclass")
+    hbdcls = ctx["hbdclass"]
     msg_to_websockets = ctx.get("msg_to_websockets")
     DEBUG = ctx.get("DEBUG", 0)
 
@@ -459,10 +463,8 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
         newh = False
 
     cid = msg.get("id", 0)
-    try:
-        rtt = float(msg.get("rtt"))
-    except TypeError:
-        rtt = None
+    rtt_raw = msg.get("rtt")
+    rtt = float(rtt_raw) if rtt_raw is not None else None
 
     if msg.get("ID") == "HTB":
         host.doesack = msg.get("acks", -1)