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:

1. Offsets were correctly NOT committed for rejected messages (preventing duplicate processing)
2. 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

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[yoelk]

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

[yoelk]

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