[BUG] Kafka producer sends a junk record into the log topic on every init

## Description
During `initClient0`, a `ProducerRecord` is constructed via the 3-arg `(topic, key, value)` constructor with **both key and value set to `StringSerializer.class.getName()`**, then `producer.send(record)` is invoked. This is the serializer class-name string, not a log payload. Each per-selector client created in multi-client mode sends one such record into the configured log topic.

## Location
```
shenyu-plugin-logging-kafka/.../client/KafkaLogCollectClient.java:99-102
```

## Impact
Downstream consumers expecting JSON `ShenyuRequestLog` records hit a parse failure on these junk messages; the log topic is polluted on every config refresh.

## Suggested fix
Remove the warm-up `send` entirely (or use `producer.partitionsFor(topic)` to validate connectivity without producing a data record).

## Related existing issue(s)
None

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Open shenyu-plugin-logging-kafka/.../client/KafkaLogCollectClient.java around lines 99-102 and trace the warm-up logic in initClient0. Confirm that client initialization no longer produces a serializer-class-name record, while configured log messages remain valid ShenyuRequestLog records and the log topic is not polluted on refresh.

Written by the indexing model from the issue text.