Allow sliding windows + combine with hot key fanout

[claudevdm]

[Please add a meaningful description for your change here
]

#20528

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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 addresses a critical limitation in Apache Beam's Python SDK by enabling the CombinePerKey.with_hot_key_fanout transform to function correctly with SlidingWindows. The core change involves a targeted modification to how windowing strategies are applied internally for hot keys under sliding windows, ensuring proper accumulation behavior without re-evaluating window assignments. This enhancement significantly improves the flexibility and robustness of combining operations, particularly in streaming pipelines that utilize complex windowing schemes.

Highlights

• Sliding Windows with Hot Key Fanout: Enabled CombinePerKey.with_hot_key_fanout to work correctly with SlidingWindows by implementing a specialized windowing strategy application that avoids incorrect window re-assignments.
• Error Removal: Removed the ValueError that was previously raised when attempting to combine SlidingWindows with hot key fanout, allowing these operations to proceed.
• Expanded Test Coverage: Added new tests to verify the correct behavior of CombineGlobally with SlidingWindows and SessionWindows when combined with hot key fanout.

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[claudevdm]

R: @tvalentyn

[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

[tvalentyn]

line 589-582 - let's move this to PR description