Introduce jitter for auto create partition

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Motivation

Now, our auto create partition machanism will create may partitions at one point time which may bring mich pressure at one time. We should introduce a jitter so that different tables may create table at different time point to reduce the presure.

Now, 12 tables, 512 bucket per partition, at around 0:10, Fluss with create them, then the isr changes quickly, in bytes decrease, and metrics will looks like:


### Solution
We can introduce jitter to alleviate it first.. But we still need to figure out why it cause isr changes quickly.

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start by locating the auto-create partition mechanism and the scheduling or trigger path that creates partitions for multiple tables. Trace how creation times are selected and how partition creation affects ISR and metrics; done means introducing a jitter that spreads table creation over time without breaking partition creation behavior, with coverage for the timing logic.

Written by the indexing model from the issue text.