Cherry-pick #36574 into release 2.69

[shunping]

Cherry-pick #36574 to fix flakiness in data generation.

[gemini-code-assist]

Summary of Changes

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This pull request addresses flakiness in data generation within the Apache Beam PeriodicSequence transform. It refines the timestamp calculation logic to guarantee that the start and end timestamps for rebased elements are perfectly synchronized, thereby enhancing the reliability of data generation processes.

Highlights

• Timestamp Consistency: Ensured that the start and end timestamps in PeriodicSequence's REBASE_ALL mode are derived from a single call to Timestamp.now(), preventing potential flakiness due to slight time differences between two separate calls.
• Walrus Operator Usage: Introduced the Python walrus operator (:=) to assign the result of Timestamp.now() to a variable now within a lambda function, allowing its reuse for both start and end timestamp calculations.

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[shunping]

R: @damccorm @Abacn @Amar3tto

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers

[shunping]

We will merge this if we are having 2.69 RC4. @Abacn

[shunping]

Since 2.69 has been released, this is no longer relevant.