fix(runtime): distinguish request parse errors from handler JSON errors

The broad try/except around _handle_invocation caught json.JSONDecodeError
from both request.json() (inbound payload) and the user's handler logic,
reporting both as "Invalid JSON in request" with HTTP 400. This misled
engineers into debugging caller payloads when the root cause was downstream
application/model-output parsing.

Narrow the JSONDecodeError and UnicodeDecodeError catches to only cover
request body parsing. Handler-raised exceptions now fall through to the
generic Exception handler (HTTP 500) with an accurate error message.

Constraint: Runtime contract returns 4xx for request-level errors only
Rejected: Catch handler JSONDecodeError separately with 422 | no precedent in runtime contract
Confidence: high
Scope-risk: narrow
Not-tested: Streaming handlers that raise JSONDecodeError mid-stream

Author: jesseturner21

src/bedrock_agentcore/runtime/app.py

@@ -537,6 +537,16 @@ async def _handle_invocation(self, request):
 
         try:
             payload = await request.json()
+        except json.JSONDecodeError as e:
+            duration = time.time() - start_time
+            self.logger.warning("Invalid JSON in request (%.3fs): %s", duration, e)
+            return JSONResponse({"error": "Invalid JSON", "details": str(e)}, status_code=400)
+        except UnicodeDecodeError as e:
+            duration = time.time() - start_time
+            self.logger.warning("Invalid encoding in request (%.3fs): %s", duration, e)
+            return JSONResponse({"error": "Invalid encoding", "details": str(e)}, status_code=400)
+
+        try:
             self.logger.debug("Processing invocation request")
 
             if self.debug:
@@ -581,15 +591,6 @@ async def _handle_invocation(self, request):
             # Use safe serialization for consistency with streaming paths
             safe_json_string = self._safe_serialize_to_json_string(result)
             return Response(safe_json_string, media_type="application/json")
-
-        except json.JSONDecodeError as e:
-            duration = time.time() - start_time
-            self.logger.warning("Invalid JSON in request (%.3fs): %s", duration, e)
-            return JSONResponse({"error": "Invalid JSON", "details": str(e)}, status_code=400)
-        except UnicodeDecodeError as e:
-            duration = time.time() - start_time
-            self.logger.warning("Invalid encoding in request (%.3fs): %s", duration, e)
-            return JSONResponse({"error": "Invalid encoding", "details": str(e)}, status_code=400)
         except HTTPException as e:
             duration = time.time() - start_time
             # Use error level for 5xx to match the generic Exception handler's severity,