Spark Runner : Replace queueStream with custom DStream in Spark streaming Flatten transform

[twosom]

Please add a meaningful description for your change here

fixes #18144
fixes #20426

This PR replaces Spark's queueStream with a custom SingleEmitInputDStream implementation
for the Flatten transform in Spark streaming. The queueStream was not checkpoint-able,
which caused issues when recovering pipelines with flattened bounded and unbounded PCollections.

The custom SingleEmitInputDStream ensures:

• Proper checkpoint recovery for Flatten transform
• Single emission of bounded data in streaming context
• Reliable handling of mixed bounded/unbounded PCollections

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[twosom]

The failing tests are also failing in the latest master branch, unrelated to this PR's changes.

[Abacn]

Thanks, spark runner tests are currently broken (by #33574) and we are working on Fix.

[Abacn]

The tests are fixed on HEAD. Would you mind rebasing your PR onto the latest master branch? Thanks!

[twosom]

The tests are fixed on HEAD. Would you mind rebasing your PR onto the latest master branch? Thanks!

@Abacn Thank you. The test runs smoothly!

[Abacn]

Thank you!