fix(kafka source): prevent data loss on rejected messages with acknowledgements

[yoelk]

Summary

Fixes data loss in the Kafka source when acknowledgements are enabled and message delivery fails.

Problem

When a message delivery fails (e.g., due to authentication errors) with acknowledgements enabled, the Kafka source would skip the failed message instead of retrying it, causing data loss.

The issue occurred because:

• Offsets were correctly NOT committed for rejected messages (preventing duplicate processing)
• But the consumer didn't seek back to retry them (causing data loss)

Solution

This PR ensures rejected messages are retried by:

• Initializing last_committed_offset on the first message to enable retry even for the first message
• Tracking acknowledgement status and setting a seek-back flag on rejection
• Preventing new message consumption while seeking back to retry
• Using a dedicated seek_to_retry_offset() method with proper error handling
• Using configured timeouts (socket_timeout_ms for seek operations, fetch_wait_max_ms for retry delay)

Changes

• Add last_committed_offset tracking to remember last successful commit
• Add need_seek_back flag to trigger retry mechanism
• Enhance acknowledgement handling to differentiate Delivered vs Errored/Rejected
• Add guard to prevent consuming new messages during seek-back
• Initialize offset tracking on first message
• Add seek_to_retry_offset() method for clean separation of concerns
• Add integration test seeks_back_on_rejected_message to validate the fix

Testing

• ✅ All existing Kafka unit tests pass (5/5)
• ✅ New integration test seeks_back_on_rejected_message validates the retry mechanism
• ✅ Clean build with no warnings
• ✅ Clippy passes with no warnings
• ✅ Manually tested with reproduction scenario (authentication failures)

The integration test verifies:

• Messages are retried after rejection
• All messages are eventually received
• Offsets are committed correctly after retry

Checklist

• Code follows Vector's style guidelines
• Tests added to validate the fix
• Documentation updated (inline comments)
• No breaking changes
• Conventional commit format used

Fixes #24543

[yoelk]

I have read the CLA Document and I hereby sign the CLA