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/client/plugins/nagios_runner.py

@@ -124,7 +124,7 @@ class NagiosRunnerPlugin(MonitorPlugin):
         Returns:
             Dictionary with results from all plugins
         """
-        results = {}
+        results: Dict[str, Any] = {}
 
         for cmd_config in self.commands:
             name = cmd_config.get("name")
@@ -181,7 +181,8 @@ class NagiosRunnerPlugin(MonitorPlugin):
                 self.logger.error(f"Command timed out: {command}")
                 return NAGIOS_UNKNOWN, f"Command timed out after {self.timeout}s", {}
 
-            status_code = proc.returncode
+            # communicate() has finished, so the process has exited and returncode is set
+            status_code = proc.returncode if proc.returncode is not None else NAGIOS_UNKNOWN
 
             if status_code < 0:
                 return NAGIOS_UNKNOWN, f"Process killed by signal {-status_code}", {}
@@ -225,7 +226,7 @@ class NagiosRunnerPlugin(MonitorPlugin):
         Returns:
             Dictionary of metric_name: value
         """
-        perfdata = {}
+        perfdata: Dict[str, Any] = {}
 
         # Performance data comes after the pipe character
         if '|' not in output: