Fix UnicodeDecodeError with non-UTF-8 filenames in SSH operations

[fabi200123]

Fix UnicodeDecodeError with non-UTF-8 filenames in SSH operations

This PR fixes a critical bug where migrations fail with UnicodeDecodeError
when source systems contain files or directories with non-UTF-8 characters in
their names.

The fix centralizes UTF-8 decoding with error handling in exec_ssh_cmd(),
which now returns decoded strings with errors='replace' instead of raw bytes.
Invalid UTF-8 sequences are replaced with the � character, allowing migrations
to continue while logging warnings for monitoring.

Changes:

• Modified exec_ssh_cmd() to return strings with safe decoding
• Added SFTP fallback in list_ssh_dir() for encoding errors
• Removed all .decode() calls on command execution results (15 locations)
• Updated test expectations to match new return type
• Preserved SFTP file reading operations (still return bytes)

Impact:

• Prevents migration failures on systems with non-UTF-8 filenames
• No breaking changes to external APIs
• Minimal performance overhead
• Backward compatible (except test mocks)