Spark Runner : Fix invalid translateImpulse

[twosom]

Please add a meaningful description for your change here

fixes #20426

This PR addresses an issue in the translateImpulse method of the SparkStreamingPortablePipelineTranslator, which was introduced in the changes made under #34080.

The root cause of the test failures was the use of ConstantInputDStream instead of SingleEmitInputDStream. This incorrect implementation led to the impulse being emitted multiple times, causing unexpected behavior.

By replacing ConstantInputDStream with SingleEmitInputDStream, this issue has been resolved.

related PR : #34253

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[twosom]

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[Abacn]

Thank you for quick fix!

btw, would you mind add a statement in https://github.com/apache/beam/blob/master/CHANGES.md#2640---unreleased to note the Spark runner improvement made here and in #34080 ?

[Abacn]

Java Tests failed at uploading logs, infra flakiness, not related to the change

[twosom]

Thank you for quick fix!

btw, would you mind add a statement in https://github.com/apache/beam/blob/master/CHANGES.md#2640---unreleased to note the Spark runner improvement made here and in #34080 ?

Thank you for pointing that out!

I completely forgot to update the CHANGES.md with the changes. I'll make sure to add the note regarding the Spark runner improvement made here and in #34080. Let me update it and create a pull request shortly.