[core] Fix dynamic bucket allocation across partitions

[ArnavBalyan]

Purpose

• Dynamic bucket assigners track the maximum bucket ID across all partitions.
• Once one partition reaches dynamic-bucket.max-buckets, new partitions incorrectly - cannot create their remaining buckets and send all rows to the first available bucket. (this is incorrect and causes skew)
• Track bucket upper bound for each partition so every partition can independently allocate its available buckets instead of having a global counter.

Tests

• UT

[JingsongLi]

+1