transfers: Prevent receiver directory wiping and data loss (#240)

[ronitgupta138]

Resolves #240

Problem & Root Cause

When transferring files or folders, FileReceiver.clean_existing_files() called rm_any() which executed shutil.rmtree() on the whole directory self.save_path / name before receiving folder contents. This was originally done because Gio.File.create() raises G_IO_ERROR_EXISTS if a destination file exists.
Additionally, when an interrupted transfer stopped or aborted prematurely, clean_current_top_dir_file() resolved abs_top_dir = self.save_path.joinpath(relative.parts[0]) and executed self.rm_any(abs_top_dir) with shutil.rmtree(), permanently destroying the entire top-level directory.

These behaviors caused severe, unintentional data loss:

1. When receiving a folder that already existed on the receiving device, any pre-existing files not present in the incoming batch (e.g. deleted or moved on sender, or transferring an update) were completely wiped out.
2. If a transfer was interrupted or cancelled mid-transfer, the entire existing receiving directory was deleted.

Solution

1. Safe Overwriting in receive_data(): Switched from self.current_gfile.create() to self.current_gfile.replace(None, False, Gio.FileCreateFlags.REPLACE_DESTINATION, None). Incoming files now overwrite cleanly without needing directory destruction.
2. Preserve Directory Trees in clean_existing_files() & rm_any(): Refactored rm_any() and clean_existing_files() to never delete directory trees (shutil.rmtree). If a target entry is a directory, it is kept intact so that existing files are preserved and transfers behave as copy operations rather than destructive overwrites.
3. Safe Abort Cleanup in clean_current_top_dir_file(): Track self.current_is_new and ensure only newly created partial files are unlinked on abort, rather than nuking the parent directory tree abs_top_dir.

Verification Steps

• Created comprehensive regression unit tests covering:
  • Preserving pre-existing directories and files when clean_existing_files() runs.
  • Verifying top-level regular files/symlinks are still handled properly.
  • Clean overwriting of individual files during transfers while leaving non-transferred files in the folder intact.
  • Aborting mid-transfer: verifying newly created partial files are removed while existing directory hierarchies and pre-existing files remain untouched.
  • Full end-to-end simulation of Issue Warpinator may delete files on receiver after interrupted transfer #240 (transferring a subset of a previously transferred directory) confirming no data loss occurs on the receiver.
• Verified all tests pass cleanly.

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