fix: prevent NPE when HTTP response fails in KsqlTarget

[Parag Badani (pbadani)]

Summary

• Fixed NullPointerException in KsqlTarget.execute() when the Vert.x HTTP response handler fires with a failed async result
• response.result() returns null on failure, but code was falling through to responseHandler.accept(response.result(), vcf), causing NPE at KsqlTarget.java:360 (resp.statusCode())
• Added return after vcf.completeExceptionally(response.cause()) to exit the handler early on failure

Root Cause

httpClientRequest.response(response -> {
    if (response.failed()) {
        vcf.completeExceptionally(response.cause());
        // missing return — fell through to responseHandler.accept(null, vcf)
    }
    responseHandler.accept(response.result(), vcf); // NPE: response.result() is null
});

Test plan

• Existing unit tests pass
• Manually verify that connection failures produce a proper KsqlRestClientException instead of an NPE

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Pull request overview

This PR hardens the Vert.x HTTP response handling in KsqlTarget.execute() to avoid a NullPointerException when the response callback is invoked with a failed AsyncResult.

Changes:

• Add an early return after completeExceptionally(...) when response.failed() is true, preventing fall-through to responseHandler.accept(response.result(), ...).

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