Refactor Queue Handling in segment_destination.dart

[neelkanth-kaushik]

Context: The original sendEvents function dequeued all events after each batch attempt, regardless of whether the batch was successfully sent or not. This led to failed events being prematurely removed from the queue, preventing retry on subsequent flush attempts.

Fix:

1. Updated the logic to only add batches to sentEvents if they are successfully sent.
2. Moved the call to _queuePlugin.dequeue(sentEvents) outside the batch loop, ensuring that only the successfully sent events are dequeued once, after all batch attempts have completed.
3. This change ensures failed events remain in the queue and are available for retry on the next flush attempt, which aligns with the expected behavior outlined in the issue.

Impact: More reliable event delivery. Failed events will no longer be silently dropped and will be retried as intended, improving data integrity and consistency.

This Pull Request contains the changes suggested in following Pull Requests:

1. Fix: Prevent failed events from being dequeued #140
2. Refactor: Move dequeue outside loop for efficiency #142