feat: add dead-letter when reach max retries

[dwego]

This pull request implements a "dead-letter" mechanism for failed log batches in the sink, ensuring that logs are not lost if delivery to the server fails after all retries. It also updates error messages to English and adds a test to verify the new behavior.

Dead-letter handling for failed batches:

• Added a write_dead_letter function to src/sink.rs that writes failed log batches to a logtap.failed.jsonl file after all retry attempts are exhausted, instead of dropping them. This ensures failed logs can be recovered or replayed later. [1] [2]
• Updated the retry logic in flush_with_retry to call write_dead_letter and log the file path when abandoning a batch.

Error message and logging improvements:

• Changed all error and status messages from Portuguese to English for consistency and clarity. [1] [2]

Testing:

• Added a test, sink_writes_batch_to_dead_letter_file_after_exhausting_retries, in tests/sink_test.rs to verify that failed batches are correctly written to the dead-letter file.