Fix flakiness in tests by increasing timeout and changing windowing type

[stankiewicz]

SpannerChangeStreamOrderedWithinKeyGloballyIT - test was flaky as it was assuming 4 distinct Spanner database writes can always happen within 2-second duration. if some write happened later as part of data preparation, dataflow was firing timer too early during the test.
SpannerChangeStreamOrderedWithinKeyIT - When FixedWindow is used, CI may not be lucky and commit one transaction at 23:59:59 and another at 00:00:01 which will end up in 2 different windows after GBK.
Moving to session window with gap to ensure that gbk will materialize complete data.

Fixes #37777

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Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request aims to enhance the reliability of Spanner change stream integration tests by mitigating flakiness. It achieves this by adjusting test parameters to be more robust against timing variations and by refining the windowing strategy in one of the tests to correctly handle data that might span across traditional fixed window boundaries.

Highlights

• Test Flakiness Resolution: Addressed flakiness in the SpannerChangeStreamOrderedWithinKeyGloballyIT test by increasing the timeIncrementInSeconds timeout from 2 to 10 seconds, providing more buffer for Spanner writes and preventing premature timer firing.
• Windowing Strategy Update: Resolved flakiness in the SpannerChangeStreamOrderedWithinKeyIT test by switching from FixedWindows to Sessions.withGapDuration for windowing, ensuring complete data materialization even when transactions span across traditional fixed window boundaries.

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[github-actions]

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[stankiewicz]

assign set of reviewers

[github-actions]

Assigning reviewers:

R: @ahmedabu98 for label java.

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[stankiewicz]

Run Java_GCP_IO_Direct PreCommit

[damccorm]

Thanks!