Properly handle timestamp prefixing of unkown window types.

[robertwb]

This was exposed by #28972 when the set of "known" coders was inadvertently reduced.

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[robertwb]

R: @Abacn

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[shunping]

Gentle ping on the review request. @Abacn

[Abacn]

tested test_windowing_before_sql locally and passed

For curiosity, what caused the window being "unknown" for that specific test?

And after this change, it seems the window is still "unknown" but the timestamp value is accessible, or I understood it incorrectly?

[Abacn]

I see, the bytes encountered in XVR_Direct test was due to here

[robertwb]

tested test_windowing_before_sql locally and passed

For curiosity, what caused the window being "unknown" for that specific test?

Due to the environment nesting it didn't realize that "interval window" type was understood by all SDKs, so fell back to the generic/unknown wrapping.

And after this change, it seems the window is still "unknown" but the timestamp value is accessible, or I understood it incorrectly?

Yep, that's exactly it.

[Abacn]

It seems this PR breaks Samza and Spark VR PostCommit, https://github.com/apache/beam/actions/workflows/beam_PostCommit_Python_ValidatesRunner_Samza.yml?query=event%3Aschedule and https://github.com/apache/beam/actions/workflows/beam_PostCommit_Python_ValidatesRunner_Spark.yml?query=event%3Aschedule

though Flink and Dataflow VR PostCommit remains green

Error message

test_custom_window_type (apache_beam.runners.portability.spark_runner_test.SparkRunnerTest) failed

E       RuntimeError: Pipeline test_custom_window_type_1710372598.7745032_87e88bc1-0f78-497a-b23a-02d9a534d8ac failed in state FAILED: java.lang.IllegalArgumentException: Unknown or unsupported WindowFn: beam:window_fn:pickled_python:v1