This is an automated cherry-pick of #19240

What is changed and how it works?

Issue Number: ref #17283, Close #19239

What's Changed:

sst_importer: enhance failpoint handling and ensure cleanup of temporary files on error

- Ensure temporary files are cleaned up when a download fails in sst_importer
- Add a new failpoint `download_files_ext_after_download` to inject errors right after batch downloads complete, improving test coverage of cleanup paths

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test (extends tests to verify temporary files and encryption metadata are cleaned up)
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Release note

Fix a bug where temporary SST files could be left behind when a download failed in sst_importer.