[I] [MSE] Support sender-side sorting for sort exchanges [pinot]

["xiangfu0 (via GitHub)" <gi...@apache.org>]

xiangfu0 opened a new issue, #19395:
URL: https://github.com/apache/pinot/issues/19395

   ## Problem
   
   `PinotLogicalSortExchange` currently sorts only on the receiver. Operators such as WINDOW and MATCH_RECOGNIZE therefore require the receiver to buffer and sort the complete exchanged input instead of merging already sorted sender streams.
   
   ## Proposed scope
   
   - Add sender-side sorting using the exchange collation, including direction and null ordering.
   - Merge sorted mailbox streams incrementally on the receiver with bounded memory.
   - Preserve cancellation, deadline, backpressure, and error propagation behavior.
   - Add distributed tests covering multiple senders, duplicate keys, nulls, and empty streams.
   
   ## Context
   
   Follow-up to #19311 and the existing receiver-side-only note in `PinotWindowExchangeNodeInsertRule`.


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