What is this fixing or adding?

Fixes #5991.

On Linux the native file pickers open_filename, save_filename and open_directory shell out to kdialog or zenity via _run_for_stdout, which reads only the helper's stdout and discards its exit code. These helpers exit 0 when the user picks a path, 1 when the user cancels, and >= 2 on an error (e.g. a missing display, a malformed argument, or the binary failing to start).

Because the exit code was thrown away, an errored helper returned empty stdout that looked identical to a user cancellation: the picker silently returned None, the error was swallowed, and the existing tk fallback was never reached. The user just saw "nothing happened".

This PR adds a small _run_file_dialog helper next to _run_for_stdout that interprets the exit code and returns (handled, selection):

• exit 0 -> (True, <path>) -- a selection was made;
• exit 1 -> (True, None) -- the user cancelled (conclusive, do not fall back);
• any other exit code -> logs a warning and returns (False, None) so the caller falls through to the next backend.

Each of the three pickers now replaces its return _run_for_stdout(...) calls for both kdialog and zenity with a fall-through pattern that returns only when the helper reports a conclusive result. A broken kdialog therefore now correctly falls back to zenity, and then to tk. messagebox is intentionally left unchanged since it is not a file picker.

How was this tested?

• Added test/general/test_utils.py with mock-based tests that patch Utils.is_linux, Utils.env_cleared_lib_path, shutil.which and subprocess.run to exercise the Linux branch on any OS:
  • exit 0 returns the chosen path and never consults the next backend;
  • exit 1 (user cancel) returns None and does not fall back;
  • exit 2 (tool error) falls back to the next backend (zenity), which then succeeds.
• python -m pytest test/general/test_utils.py -q -> 3 passed.
• python -m py_compile Utils.py -> clean.
• python -m flake8 --max-line-length=120 Utils.py test/general/test_utils.py -> no new warnings on the changed lines.