Refactor ConfluentKafkaInput to store offsets for last message

This commit refactors the ConfluentKafkaInput class to store offsets for the last message referenced by `_last_valid_records`. Previously, offsets were stored for each kafka partition in `_last_valid_records`, but now only the last valid record is stored. This change improves the efficiency of offset storage and reduces memory usage.

Code changes:
- Modified `ConfluentKafkaInput` class in `logprep/connector/confluent_kafka/input.py`
- Removed `_last_valid_records` dictionary and replaced it with `_last_valid_record` variable
- Updated `batch_finished_callback` method to store offsets for the last valid record

logprep/connector/confluent_kafka/input.py

@@ -44,6 +44,7 @@
     OFFSET_STORED,
     Consumer,
     KafkaException,
+    Message,
     TopicPartition,
 )
 from confluent_kafka.admin import AdminClient
@@ -244,13 +245,13 @@ class Config(Input.Config):
 
         """
 
-    _last_valid_records: dict
+    _last_valid_record: Message
 
-    __slots__ = ["_last_valid_records"]
+    __slots__ = ["_last_valid_record"]
 
     def __init__(self, name: str, configuration: "Connector.Config") -> None:
         super().__init__(name, configuration)
-        self._last_valid_records = {}
+        self._last_valid_record = None
 
     @property
     def _kafka_config(self) -> dict:
@@ -418,7 +419,7 @@ def _get_raw_event(self, timeout: float) -> bytearray:
             raise CriticalInputError(
                 self, "A confluent-kafka record contains an error code", str(kafka_error)
             )
-        self._last_valid_records[message.partition()] = message
+        self._last_valid_record = message
         labels = {"description": f"topic: {self._config.topic} - partition: {message.partition()}"}
         self.metrics.current_offsets.add_with_labels(message.offset() + 1, labels)
         return message.value()
@@ -467,17 +468,15 @@ def _enable_auto_commit(self) -> bool:
         return self._config.kafka_config.get("enable.auto.commit") == "true"
 
     def batch_finished_callback(self) -> None:
-        """Store offsets for each kafka partition in `self._last_valid_records`.
-        Should be called by output connectors if they are finished processing
-        a batch of records.
+        """Store offsets for last message referenced by `self._last_valid_records`.
+        Should be called after delivering the current message to the output or error queue.
         """
         if self._enable_auto_offset_store:
             return
-        for message in self._last_valid_records.values():
-            try:
-                self._consumer.store_offsets(message=message)
-            except KafkaException as error:
-                raise InputWarning(self, f"{error}, {message}") from error
+        try:
+            self._consumer.store_offsets(message=self._last_valid_record)
+        except KafkaException as error:
+            raise InputWarning(self, f"{error}, {self._last_valid_record}") from error
 
     def _assign_callback(self, consumer, topic_partitions):
         for topic_partition in topic_partitions: