docs: improve dropping records example

Author: heitorlessa

docs/utilities/data_classes.md

@@ -994,10 +994,12 @@ To do that, you can use `KinesisFirehoseDataTransformationResponse` class along
 
 === "Dropping invalid records"
 
-    ```python
+    ```python hl_lines="5-6 16 34"
     --8<-- "examples/event_sources/src/kinesis_firehose_response_drop.py"
     ```
 
+    1. This exception would be generated from `record.data_as_json` if invalid payload.
+
 === "Indicating a processing failure"
 
     ```python

examples/event_sources/src/kinesis_firehose_response_drop.py

@@ -1,29 +1,33 @@
+from json import JSONDecodeError
+from typing import Dict
+
 from aws_lambda_powertools.utilities.data_classes import (
     KinesisFirehoseDataTransformationRecord,
     KinesisFirehoseDataTransformationResponse,
     KinesisFirehoseEvent,
+    event_source,
 )
 from aws_lambda_powertools.utilities.serialization import base64_from_json
 from aws_lambda_powertools.utilities.typing import LambdaContext
 
 
-def lambda_handler(event: dict, context: LambdaContext):
-    firehose_event = KinesisFirehoseEvent(event)
+@event_source(data_class=KinesisFirehoseEvent)
+def lambda_handler(event: KinesisFirehoseEvent, context: LambdaContext):
     result = KinesisFirehoseDataTransformationResponse()
 
-    for record in firehose_event.records:
+    for record in event.records:
         try:
-            payload = record.data_as_text  # base64 decoded data as str
-            ## do all kind of stuff with payload
+            payload: Dict = record.data_as_json  # decodes and deserialize base64 JSON string
+
             ## generate data to return
             transformed_data = {"tool_used": "powertools_dataclass", "original_payload": payload}
-            # Default result is Ok
+
             processed_record = KinesisFirehoseDataTransformationRecord(
                 record_id=record.record_id,
                 data=base64_from_json(transformed_data),
             )
-        except Exception:
-            # encountered failure that couldn't be fixed by retry
+        except JSONDecodeError:  # (1)!
+            # our producers ingest JSON payloads only; drop malformed records from the stream
             processed_record = KinesisFirehoseDataTransformationRecord(
                 record_id=record.record_id,
                 data=record.data,