Close remote handles after uploads

[numbereleven-a]

Summary

• Wrap SFTP upload handles and native SCP upload channels with the existing RAII helpers.
• Close the remote handle before reporting upload completion.
• Report SFTP close failures as upload write errors.
• Ensure early-return and interrupted-transfer paths also release the remote handle.

Root cause

SftpUploadFileW stored the SFTP handle and SCP channel in adapter unique_ptrs, but destroying those adapters does not close the underlying libssh2 handle.

As a result, an uploaded file could remain open for writing until the entire SFTP connection was disconnected. On Linux and OpenWrt, attempting to execute that file failed with ETXTBSY:

Text file busy

Verification

Tested against the current upstream main branch on OpenWrt 23.05.5 while keeping the SFTP connection open.

Before:

Text file busy
BEFORE_RC=126
/proc/22338/fd/3 -> /opt/test/sftp_etxtbsy_probe.sh

After:

SFTP_ETXTBSY_PROBE_OK
AFTER_RC=0

No process retained an open file descriptor for the uploaded file after the fixed transfer completed.

The patched upstream source also builds successfully in x64 Release configuration.