The restriction for continuity of DXF add index is too hard

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

1. Enable distributed DDL backfill for `ADD INDEX`.
2. Create a large table that spans many TiKV regions.
3. Execute `ALTER TABLE ... ADD INDEX ...` so that TiDB uses DXF backfill planning.
4. During backfill subtask planning, let TiKV region metadata change, for example by region merge/split or scheduling.
5. TiDB might get region metadata that is not exactly continuous and repeatedly block or fail the DDL planning.

### 2. What did you expect to see? (Required)

`ADD INDEX` should continue to generate DXF backfill subtasks. Region metadata is only used to roughly split the backfill key range into batches, so transient non-continuous region metadata should not block the DDL as long
as the generated backfill ranges still cover the original table range continuously.

### 3. What did you see instead (Required)

`ADD INDEX` could be blocked during DXF backfill range planning because TiDB required adjacent region metadata to be exactly continuous. In a large table, region merge/split or scheduling might make the region metadata
returned by RegionCache temporarily non-continuous or overlapping.

In one case, an `ADD INDEX` DDL on a large table ran for more than 26 hours before being manually canceled.

### 4. What is your TiDB version? (Required)
v8.5.4

Start at the DXF backfill range-planning path for ALTER TABLE ... ADD INDEX and inspect how RegionCache metadata is handled during region merge, split, or scheduling. Reproduce the large-table scenario with distributed DDL backfill enabled. Done means transient non-contiguous or overlapping metadata no longer blocks planning when generated ranges still cover the original table range continuously.

Written by the indexing model from the issue text.