Fix KafkaItemReader ExecutionContext deserialization error when using Jackson2ExecutionContextStringSerializer

[noojung]

When use Jackson2ExecutionContextStringSerializer, KafkaItemReader always encountered an ClassCastException.

Because KafkaItemReader stores a Map<TopicPartition, Long> in the ExecutionContext,
Jackson2ExecutionContextStringSerializer forces all map keys to String.

So I change it to store only the partition number (as String) instead of TopicPartition directly.

This change:

• Modifies update() to save only the partition number (as a String) into the ExecutionContext
• Updates open() to read those String offsets and reconstruct each TopicPartition

As a result, KafkaItemReader can now successfully deserialize ExecutionContext without throwing an exception.

Resolves: #3797