Follow on PR #15146 to reslove the backwad compatability issue.
1. /agents/<attachment_id>/download ->
/agents/attachments/<attachment_id>/download
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Bugfix: keep document api backward compatible
Fix 1: https://github.com/infiniflow/ragflow/issues/14634
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
add compatibility route for document download under /v1
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
add file convert backward compatibility
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
add legacy agent completion API compatibility
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)