fix(acp): silence 'Background task failed' noise on liveness-probe requests (#12855)

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

acp_adapter/entry.py

@@ -20,6 +20,46 @@ from pathlib import Path
 from hermes_constants import get_hermes_home
 
 
+# Methods clients send as periodic liveness probes. They are not part of the
+# ACP schema, so the acp router correctly returns JSON-RPC -32601 to the
+# caller — but the supervisor task that dispatches the request then surfaces
+# the raised RequestError via ``logging.exception("Background task failed")``,
+# which dumps a traceback to stderr every probe interval. Clients like
+# acp-bridge already treat the -32601 response as "agent alive", so the
+# traceback is pure noise. We keep the protocol response intact and only
+# silence the stderr noise for this specific benign case.
+_BENIGN_PROBE_METHODS = frozenset({"ping", "health", "healthcheck"})
+
+
+class _BenignProbeMethodFilter(logging.Filter):
+    """Suppress acp 'Background task failed' tracebacks caused by unknown
+    liveness-probe methods (e.g. ``ping``) while leaving every other
+    background-task error — including method_not_found for any non-probe
+    method — visible in stderr.
+    """
+
+    def filter(self, record: logging.LogRecord) -> bool:
+        if record.getMessage() != "Background task failed":
+            return True
+        exc_info = record.exc_info
+        if not exc_info:
+            return True
+        exc = exc_info[1]
+        # Imported lazily so this module stays importable when the optional
+        # ``agent-client-protocol`` dependency is not installed.
+        try:
+            from acp.exceptions import RequestError
+        except ImportError:
+            return True
+        if not isinstance(exc, RequestError):
+            return True
+        if getattr(exc, "code", None) != -32601:
+            return True
+        data = getattr(exc, "data", None)
+        method = data.get("method") if isinstance(data, dict) else None
+        return method not in _BENIGN_PROBE_METHODS
+
+
 def _setup_logging() -> None:
     """Route all logging to stderr so stdout stays clean for ACP stdio."""
     handler = logging.StreamHandler(sys.stderr)
@@ -29,6 +69,7 @@ def _setup_logging() -> None:
             datefmt="%Y-%m-%d %H:%M:%S",
         )
     )
+    handler.addFilter(_BenignProbeMethodFilter())
     root = logging.getLogger()
     root.handlers.clear()
     root.addHandler(handler)