Exposed source window to AssignContext in window_mapping_fn for side inputs

[alexmalao1]

Added source window to default_window_mapping_fn for side inputs. Existing window functions use timestamp to assign windows for side inputs, but custom windows not based on time would need to propagate the source window.

Addresses #36719

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[gemini-code-assist]

Summary of Changes

Hello @alexmalao1, 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 improves the flexibility of windowing for side inputs in Apache Beam by providing the full source window object to the AssignContext. This allows for more sophisticated and custom window assignment strategies that might rely on properties of the source window beyond just its maximum timestamp, addressing a limitation for non-timestamp-based windowing scenarios.

Highlights

• Enhanced Window Assignment for Side Inputs: The AssignContext within default_window_mapping_fn now explicitly exposes the source_window object, in addition to its max_timestamp.
• Support for Custom Windowing: This change enables custom window functions for side inputs that are not solely dependent on timestamps, allowing for more flexible window propagation logic.

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[github-actions]

Assigning reviewers:

R: @damccorm for label python.

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[hjtran]

Nice find @alexmalao1. Could you include a test? Probably easiest to first put up a separate PR for adding the non-time-based custom window we've implemented and then creating a test using that

[damccorm]

Nice find @alexmalao1. Could you include a test? Probably easiest to first put up a separate PR for adding the non-time-based custom window we've implemented and then creating a test using that

+1 to adding a test, otherwise this LGTM though - thanks for the find

[github-actions]

Reminder, please take a look at this pr: @damccorm

[damccorm]

waiting on author