[O11YINFRA-109] catch AssertionError from bmemcached on malformed binary protocol responses (DataDog#22650)

* fix(mcache): catch AssertionError from bmemcached on malformed binary protocol responses

The bmemcached library throws AssertionError when the binary protocol
magic byte doesn't match expectations (e.g., corrupted or partial
response under high load). This error was not caught by the check's
exception handler, causing the entire check to crash with an unhandled
AssertionError instead of reporting a proper CRITICAL service check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* handle AssertionError separately: log warning instead of raising ConfigurationError

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ivotustanic

mcache/changelog.d/22650.fixed

@@ -0,0 +1 @@
+Handle malformed binary protocol responses from `bmemcached` gracefully. Instead of crashing with an unhandled `AssertionError`, the check now logs a warning and reports a `WARNING` service check, allowing the next check run to retry.

mcache/datadog_checks/mcache/mcache.py

@@ -305,6 +305,16 @@ def check(self, instance):
                     server, port, e
                 )
             )
+        except AssertionError:
+            self.warning(
+                "Received malformed response from memcache instance %s:%s, skipping this check run", server, port
+            )
+            self.service_check(
+                self.SERVICE_CHECK,
+                AgentCheck.WARNING,
+                tags=service_check_tags,
+                message="Malformed binary protocol response",
+            )
         else:
             client.disconnect_all()
             self.log.debug("Disconnected from memcached")