ddl/dxf: precheck TiKV disk space and report actual TiKV capacity increase for add index in local sort and global sort

## Enhancement

### Background

In DXF add index, both local sort and global sort eventually write new index data into TiKV through the worker execution path.

Today, we still have two observability / safety gaps in this flow:

1. Before the TiDB worker starts executing, TiDB does not proactively check whether TiKV has enough free disk space for the incoming add-index workload.
2. After the add-index job finishes, TiDB does not report how much TiKV capacity was actually added by the job.

This means the job may start and run for a while before hitting TiKV-side disk pressure, and operators also cannot easily understand the real storage cost introduced by the add-index job.

Related background:
- #60005

### Goal

Improve DXF add-index safety and observability for both local sort and global sort by:

- adding a TiKV disk-space precheck before the TiDB worker starts execution
- reporting the actual observed TiKV capacity increase caused by the add-index job after execution