Add e2e tests for Elasticsearch source connector

[hubcio]

The Elasticsearch source connector needs integration tests. See the Postgres source tests (#2579) for the testing pattern.

What needs testing

• Basic flow: documents in ES → messages in Iggy
• JSON document handling
• Pagination modes (scroll-based, cursor-based)
• Timestamp filtering behavior
• State persistence (last_document_id, last_poll_timestamp)

Implementation notes

• Reuse the testcontainers ES setup from sink tests
• Use poll_messages pattern from postgres source tests
• Test state recovery scenarios

Files to reference

• core/integration/tests/connectors/postgres/postgres_source.rs
• core/connectors/sources/elasticsearch_source/src/lib.rs
• core/connectors/sources/elasticsearch_source/src/state_manager.rs

Acceptance criteria

• Tests for at least 2 pagination modes
• State persistence test (stop/restart scenario)
• Clear failure messages

[gobraves]

hi, @hubcio can i take this issue?

[mmodzelewski]

@gobraves go ahead, you're assigned :)