KAFKA-20751: Preserve available versions for cached connector plugin validation failures

[majialoong]

Preserve the available plugin versions stored in cached connector plugin validation failures.

This keeps repeated validation of an unavailable connector.plugin.version consistent with the initial validation response and avoids an NPE in the cached failure path.

[majialoong]

Before this change, validating the same unavailable connector plugin version twice could return a 500/NPE response instead of the normal validation result.

Request:

http://localhost:8083/connector-plugins/org.apache.kafka.connect.file.FileStreamSinkConnector/config/validate

{
    "connector.class": "org.apache.kafka.connect.file.FileStreamSinkConnector",
    "connector.plugin.version": "2.0.1", // unavailable connector plugin version
    "tasks.max": "1",
    "topics": "test"
}

The second request will receive the following response:

{
    "error_code": 500,
    "message": "java.lang.NullPointerException: Cannot invoke \"java.util.List.stream()\" because the return value of \"org.apache.kafka.connect.runtime.isolation.VersionedPluginLoadingException.availableVersions()\" is null"
}