Fix blocking I/O call in DBusManager.load() (#6346)

Wrap SOCKET_DBUS.exists() call in sys_run_in_executor to avoid blocking
os.stat() call in async context. This follows the same pattern already
used in supervisor/resolution/evaluations/dbus.py.

Fixes SUPERVISOR-11HC

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

supervisor/dbus/manager.py

@@ -115,7 +115,7 @@ class DBusManager(CoreSysAttributes):
 
     async def load(self) -> None:
         """Connect interfaces to D-Bus."""
-        if not SOCKET_DBUS.exists():
+        if not await self.sys_run_in_executor(SOCKET_DBUS.exists):
             _LOGGER.error(
                 "No D-Bus support on Host. Disabled any kind of host control!"
             )