Summary

This fixes the deprecated Gradio UI startup path on non-Windows platforms.

Problem

pyrit.ui.app currently treats the Windows mutex logic as mandatory in two places:

• is_app_running() raises NotImplementedError on non-Windows platforms
• the __main__ startup path defines a nested create_mutex() helper that also raises NotImplementedError on non-Windows platforms

That creates two failure modes on Linux/macOS:

1. AppRPCServer.start() calls is_app_running() before launching the UI, so the RPC path fails during the probe step.
2. Even if the process is launched directly, the child process fails immediately in the __main__ block before it can start the Gradio app.

Fix

• move create_mutex() to module scope so the startup guard is reusable and testable
• make create_mutex() a no-op that returns True on non-Windows platforms
• make is_app_running() return False on non-Windows platforms instead of raising

This preserves the existing Windows mutex-based single-instance behavior while allowing non-Windows platforms to proceed through the UI startup path.

Tests

Added unit coverage for the non-Windows behavior:

• is_app_running() returns False
• create_mutex() returns True

Validation command:

uv run --extra dev pytest tests/unit/ui/test_app.py -q

Validation result:

2 passed, 1 warning in 0.04s

The warning is the existing deprecation warning for pyrit.ui.