Clients like acp-bridge send periodic bare `ping` JSON-RPC requests as a
liveness probe. The acp router correctly returns JSON-RPC -32601 to the
caller, which those clients already handle as 'agent alive'. But the
supervisor task that ran the request then surfaces the raised RequestError
via `logging.exception('Background task failed', ...)`, dumping a full
traceback to stderr on every probe interval.
Install a logging filter on the stderr handler that suppresses
'Background task failed' records only when the exception is an acp
RequestError(-32601) for one of {ping, health, healthcheck}. Real
method_not_found for any other method, other exception classes, other log
messages, and -32601 logged under a different message all pass through
untouched.
The protocol response is unchanged — the client still receives a standard
-32601 'Method not found' error back. Only the server-side stderr noise is
silenced.
Closes#12529
