Use EventFactory instead of JacksonEvent.builder() in iceberg-source

[lawofcycles]

Description

Replaces direct usage of JacksonEvent.builder() with EventFactory in the iceberg-source plugin, following the standard Data Prepper pattern used by other source plugins such as FileSource. This is a follow-up to #6554.

Changes:

• Add EventFactory as a constructor parameter to IcebergSource, IcebergService, ChangelogWorker, and ChangelogRecordConverter
• Replace JacksonEvent.builder() with eventFactory.eventBuilder(EventBuilder.class) in ChangelogRecordConverter
• Update unit and integration tests accordingly

Issues Resolved

Related to #6552

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
• New functionality has javadoc added
• Commits are signed with a real name per the DCO

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[lawofcycles]

I have opened #6682 which adds source-layer shuffle to the iceberg-source plugin. It modifies ChangelogWorker and IcebergService significantly. There are no functional conflicts between the two PRs, but once #6682 is merged, this PR will need a rebase to resolve code-level conflicts in the constructor wiring in IcebergService.

[dlvenable]

@lawofcycles , let's merge this one in and then we can rebase the other.