[I] [Task]: Tighten the late data bound of the Spark 4 streaming stateful ParDo to the previous batch watermark [beam]

["tkaymak (via GitHub)" <gi...@apache.org>]

tkaymak opened a new issue, #40446:
URL: https://github.com/apache/beam/issues/40446

   ### What needs to happen?
   
   The Spark 4 Structured Streaming stateful ParDo filters late data against the watermark stored on the key's previous visit, so a key on its first visit or idle for several batches accepts data that a per batch bound would drop. Spark holds the per batch value as `eventTimeWatermarkForLateEvents` in `TransformWithStateExec`, but `StatefulProcessor` only sees the eviction watermark through `TimerValues`. Tighten the bound once Spark exposes the late events watermark, or by tracking the previous batch watermark outside keyed state.
   
   Part of #36841.
   
   ### Issue Priority
   
   Priority: 3 (nice-to-have improvement)
   
   ### Issue Components
   
   - [x] Component: Spark Runner
   


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