docs(runtime): document time_of_last_update field in /ping response

src/bedrock_agentcore/runtime/app.py

@@ -610,6 +610,14 @@ async def _handle_invocation(self, request):
             return JSONResponse({"error": str(e)}, status_code=500)
 
     def _handle_ping(self, request):
+        """Handle GET /ping — return status and last-update timestamp.
+
+        Response body: ``{"status": "Healthy|HealthyBusy", "time_of_last_update": <int>}``
+
+        Both fields are required by the platform.  ``time_of_last_update`` is a
+        Unix timestamp (seconds); without it the idle reaper fires even when
+        ``status`` is ``HealthyBusy``.  See :class:`PingStatus` for details.
+        """
         try:
             status = self.get_current_ping_status()
             self.logger.debug("Ping request - status: %s", status.value)

src/bedrock_agentcore/runtime/models.py

@@ -7,7 +7,22 @@
 
 
 class PingStatus(str, Enum):
-    """Ping status enum for health check responses."""
+    """Ping status enum for the ``GET /ping`` health-check endpoint.
+
+    The platform polls ``/ping`` every few seconds to decide whether to keep
+    the container alive.  The full JSON response body emitted by this SDK is::
+
+        {
+            "status": "<Healthy|HealthyBusy>",
+            "time_of_last_update": <unix-timestamp-int>
+        }
+
+    ``time_of_last_update`` is **required** for the idle-reaper logic.  If it
+    is absent, ``idleRuntimeSessionTimeout`` fires at the configured boundary
+    even when ``status`` is ``HealthyBusy``, silently terminating the
+    container mid-execution.  The SDK populates this field automatically; if
+    you implement a custom ping handler outside of the SDK, you must include it.
+    """
 
     HEALTHY = "Healthy"
     HEALTHY_BUSY = "HealthyBusy"