Fix non-Windows UI startup checks

[biefan]

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.

[rlundeen2]

This looks good, but we're deprecating all of this very soon, it probably should already be deprecated. The new frontend is in /frontend.

I don't see a downside to merging this sooner vs later though

[Copilot]

Pull request overview

This PR updates the deprecated pyrit.ui startup path so that non-Windows platforms (Linux/macOS) no longer error out during the “is UI already running?” probe or the __main__ startup guard, while preserving the existing Windows single-instance mutex behavior.

Changes:

• Introduces a reusable module-scope create_mutex() helper and uses it in __main__.
• Makes create_mutex() a no-op returning True on non-Windows platforms.
• Makes is_app_running() return False on non-Windows platforms instead of raising, and adds unit tests for these behaviors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pyrit/ui/app.py	Moves mutex creation to module scope and makes both mutex creation and “already running” checks non-fatal on non-Windows.
tests/unit/ui/test_app.py	Adds unit tests asserting non-Windows behavior for is_app_running() and create_mutex().

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