[Umbrella] Rescale buckets for future partitions

### Search before asking

- [x] I searched the existing issues and found no umbrella issue tracking this work.

### Description

This umbrella issue tracks support for changing the bucket count of future partitions, together with the recovery coverage and client improvements needed around this feature.

Changing a partitioned table's `bucket.num` should affect only partitions created after the change. Existing partitions retain their original bucket counts, without data redistribution or lake-file rewrites. Reads, writes, and recovery must therefore work correctly when partitions of the same table have different bucket counts.

### Sub-issues

- #3907 — Core support for per-partition bucket rescaling, implemented in #3908: persist and propagate each partition's bucket count and use it for routing while allowing the table default to change for future partitions.
- #4280 — Add aggregation recovery tests across partitions with different bucket counts. Cover both restoration from a real aggregation checkpoint and recovery through producer offsets without a checkpoint, including another failure during recovery. Exercise multiple operator subtasks and verify exact aggregate results with no duplicate application or data loss.
- #4287 — Keep dynamic partition writes non-blocking with asynchronous partition creation and metadata refresh. Use fresh partition metadata when forming new batches and validate each queued batch's original routing at send time, failing incompatible batches rather than silently changing their routing.
- #4289 — Add feature introduction and usage documentation, including runnable examples for increasing and decreasing future partitions' bucket counts, existing/pre-created partition semantics, and supported configurations and compatibility requirements.

### Delivery scope

The core feature is tracked by #3907 and PR #3908. The recovery tests, asynchronous write-path improvement, and user documentation are tracked independently in the sub-issues above. In particular, #4287 is a follow-up to be addressed in a separate PR after #3908; it is not required to be implemented within #3908.

This umbrella remains open until all sub-issues are completed.

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR.

Start by reading the linked core work in #3907 and implemented PR #3908, then choose one independent sub-issue: #4280 for aggregation recovery tests, #4287 for asynchronous dynamic-partition writes, or #4289 for feature documentation. Read the selected sub-issue first and verify completion against its stated recovery, routing, or documentation requirements.

Written by the indexing model from the issue text.