[Feature] Reduce redundant shuffle for spark dynamic bucket writes

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Motivation

Dynamic bucket writing does two shuffles, the first `repartitionByKeyPartitionHash` seems unnecessary, It seems to be only used to determine `assignId`. However, `assignId` can be calculated through `partitionHash/keyHash/numParallelism/numAssigners`, we do not need to do extra shuffle. Can we remove it?

https://github.com/apache/paimon/blob/e27ceb464244f5a0c2bfa2a7c6db649ca945212b/paimon-spark/paimon-spark-common/src/main/scala/org/apache/paimon/spark/commands/PaimonSparkWriter.scala#L143

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Read paimon-spark/paimon-spark-common/src/main/scala/org/apache/paimon/spark/commands/PaimonSparkWriter.scala around line 143, then trace dynamic bucket writing and how assignId is derived. Confirm that the proposed partitionHash/keyHash/numParallelism/numAssigners calculation preserves bucket assignment, and that the first repartitionByKeyPartitionHash can be removed without changing write behavior.

Written by the indexing model from the issue text.