[Bug] Snapshot creation fails with hard link NOT_FOUND error when binlog enabled during CLONE task

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Version

- Doris Version: 2.1.8

### What's Wrong?

## Error Message (from be.WARNING)
[NOT_FOUND]failed to create hard link from
/mnt/storage00/doris/be/storage/data/698/924175/200549890/_binlog/xxx.dat
to
/mnt/storage00/doris/be/storage/snapshot/xxx/924175/200549890/xxx.binlog

## Key Observations
1. Source binlog files (.dat and .idx) exist in _binlog/ directory
2. data/ and snapshot/ directories are on the same filesystem (/dev/nvme0n1)
3. Multiple BEs all fail with the same error
4. snapshot directory is never created due to hard link failure
5. The requested file extension in clone URL is .binlog-index
but actual file on disk is .idx — possible filename mapping bug

## Steps to Reproduce
1. Enable binlog on a table
2. Execute DECOMMISSION BACKEND
3. Observe CLONE tasks fail repeatedly with above hard link error

### What You Expected?

When binlog is enabled on a table and a CLONE task is triggered
(e.g., during DECOMMISSION BACKEND), the snapshot creation should
successfully create hard links for binlog files (.dat -> .binlog),
allowing the CLONE task to complete normally.

The snapshot directory should be created with all required files,
and the target BE should be able to download them without 404 errors.

### How to Reproduce?

_No response_

### Anything Else?

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)