xkafka: Producer.AsyncPublish causes goroutine leak when Start/Run is not called

Nobody has claimed this yet.

## Summary

The `Producer` struct requires `Start()` or `Run()` to be called to drain the internal `events` channel (delivery reports). However, `AsyncPublish` works without `Start()` being called — it silently pushes messages via `ProduceChannel()` without any compile-time or runtime indication that the event loop is missing.

## Problem

When `Start()` is not called:
1. The `events` channel ([producer.go#L17](https://github.com/gojekfarm/xtools/blob/main/xkafka/producer.go#L17)) fills up with unhandled delivery reports
2. The confluent-kafka-go internal poller blocks because it cannot push to the full events channel
3. `ProduceChannel()` backs up because the poller is blocked
4. Any goroutine calling `AsyncPublish` blocks forever on `p.kafka.ProduceChannel() <- km`
5. This results in an unbounded goroutine and memory leak

## Reproduction

Call `NewProducer()` followed by `AsyncPublish()` repeatedly without ever calling `Start()` or `Run()`. Observe goroutine count growing without bound.

## Suggested Improvements

- **Document the requirement**: The current doc on `Start()` only says "starts kafka event handling" without explaining it is mandatory for `AsyncPublish` to function correctly. Add explicit documentation that `Start()`/`Run()` must be called when using `AsyncPublish`.
- **Return an error**: Consider returning an error from `AsyncPublish` if `Start()` has not been called yet.
- **Auto-start**: Alternatively, auto-start the event loop lazily on the first `AsyncPublish` call via `sync.Once`.

Read xkafka/producer.go around the events channel and the Start, Run, and AsyncPublish paths. Reproduce repeated AsyncPublish calls without Start or Run and observe the blocked producer behavior and goroutine growth. Done requires an agreed lifecycle behavior, clear documentation or API handling, and verification that AsyncPublish does not block indefinitely when the event loop is missing.

Written by the indexing model from the issue text.