xftp: recreate work directory if missing before creating transfer prefix path

[Narasimha-sc]

Problem

xftpSendFile', xftpSendDescription' and xftpReceiveFile' create their per-transfer work directory with a non-recursive createDirectory on the path from getPrefixPath, which assumes the configured XFTP work directory already exists. If that directory is removed while the app is running (third-party temp/disk cleaner, roaming-profile sync, or manual deletion), every subsequent transfer fails and cannot recover until the app is restarted.

Reported from simplex-chat desktop (Windows):

CreateDirectory "\\?\C:\Users\<user>\AppData\Roaming\SimpleX\tmp\20260701_231208_256584_snd.xftp": does not exist

...\Roaming\SimpleX\tmp is the work directory; it is created once at startup but never re-created before a transfer.

Root cause

src/Simplex/FileTransfer/Agent.hs:

getPrefixPath suffix = do
  workPath <- getXFTPWorkPath           -- not guaranteed to still exist
  ...
  uniqueCombine workPath (isoTime <> "_" <> suffix)

-- callers:
prefixPath <- lift $ getPrefixPath "snd.xftp"
createDirectory prefixPath              -- non-recursive; throws if workPath is gone

Fix

Ensure the work directory exists in getPrefixPath with createDirectoryIfMissing True. This is a single point that covers all three transfer entry points (send, send-description, receive) and the chunk/tmp paths built from getXFTPWorkPath via toFSFilePath. The individual createDirectory prefixPath calls are unchanged — uniqueCombine still yields a fresh name.

getPrefixPath suffix = do
  workPath <- getXFTPWorkPath
  createDirectoryIfMissing True workPath
  ...

Testing

Added a regression test (should recreate work directory removed while running before sending) that removes the agent work directory after workers start, then sends a file.

Verified red→green locally (GHC 9.6.3), running only the new test:

• Without the fix: 1 example, 1 failure — createDirectory: does not exist on …/work/<ts>_snd.xftp (reproduces the reported error).
• With the fix: 1 example, 0 failures.

The full /XFTP/XFTP agent/ group also passes with the fix (27 examples, 0 failures, 1 pending), confirming no regression to normal transfers.

Notes

• createDirectoryIfMissing is already in scope (import UnliftIO.Directory).
• Non-recursive createDirectory calls that build paths inside an already-created prefixPath are intentionally left as-is; their parent exists by construction.