[MSE] Support sender-side sorting for sort exchanges

[xiangfu0]

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.