[fix](local shuffle) fix bucket shuffle exchange incorrectly marked as serial in pooling mode

[924060929]

1. ExchangeNode.isSerialOperator(): move useSerialSource() guard into isSerialOperator()
      so that is_serial is a self-contained per-operator property. Only UNPARTITIONED or
      use_serial_exchange exchanges in pooling mode are serial. Bucket shuffle / hash
      exchanges are never serial — scan seriality is handled by the scan pipeline independently.

2. DistributePlanner.filterInstancesWhichCanReceiveDataFromRemote(): use
      linkNode.isSerialOperator() to decide whether to dedupe to first-per-worker.
      Remove LocalShuffleAssignedJob type check and enableShareHashTableForBroadcastJoin
      parameter — serial exchange semantics already capture the correct behavior.

3. Add unit tests for both changes.

Issue Number: close #xxx

Related PR: #xxx

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[Thearas]

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