[I] [spark] Port TreeMap-collapse log buffer to FlussUpsertPartitionReader [fluss]

["fresh-borzoni (via GitHub)" <gi...@apache.org>]

fresh-borzoni opened a new issue, #3320:
URL: https://github.com/apache/fluss/issues/3320

   ### Search before asking
   
   - [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.
   
   
   ### Description
   
   FlussUpsertPartitionReader.createLogChangesIterator() materializes the full log tail in mutable.ArrayBuffer and sort before merging. So memory grows with total log records.
   
   LakeSnapshotAndLogSplitScanner.pollLogRecords() already implements the right pattern - TreeMap keyed by PK with eager per-PK collapse on put. So memory grows with distinct PKs.
   
   This is the better pattern and more unified one, so let's do it.
   
   ### Willingness to contribute
   
   - [x] I'm willing to submit a PR!


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