Fix KafkaItemReader ClassCastException during ExecutionContext deserialization

[noojung]

Using Jackson2ExecutionContextStringSerializer causes KafkaItemReader to throw a ClassCastException.

Because KafkaItemReader stores a Map<TopicPartition, Long> in the ExecutionContext, the serializer converts all map keys to strings.

To fix this, I changed it to store only the partition number (as a string) instead of the full TopicPartition object.

Resolves: #3797

[fmbenhassine]

Using Jackson2ExecutionContextStringSerializer causes KafkaItemReader to throw a ClassCastException.

Jackson2ExecutionContextStringSerializer is deprecated in favor of JacksonExecutionContextStringSerializer (based on Jackson 3, see #4842). Can you please try with the new serializer and see if the issue persists?

[noojung]

@fmbenhassine I've tested it, and the same ClassCastException still occurs with the new serializer.

I've written a sample test case on my branch: noojung@c91f775

Please let me know if you have any feedback.

[fmbenhassine]

LGTM 👍 Rebased and merged as 3080a6e. Thank you for your contribution!