Fix Windows subprocess timeouts with CREATE_NO_WINDOW flag

The Problem

Windows CI tests have been timing out consistently. Tests like test_lazy_auto_backend_selection, test_interactive_backend, test_fontcache_thread_safe, and many others fail at the 20-second timeout mark. This happens across Python 3.11, 3.12, and 3.13 on Windows.
The common pattern: all these tests spawn subprocesses.

Why It Happens

On Windows, when you create a subprocess, the OS tries to create a console window for it by default. Even though we're running headless tests in CI, Windows still goes through this window creation process. This adds 1-3 seconds of overhead to each subprocess call.
When you have multiple tests spawning subprocesses, these delays add up quickly and push tests over the 20-second timeout limit.

The Fix

Python provides a flag specifically for this: subprocess.CREATE_NO_WINDOW. It tells Windows "don't bother creating a console window for this process."
I added this flag to our subprocess_run_for_testing() helper function, so it automatically applies to all tests that use it:

This way:

It only applies on Windows (no impact on Linux/macOS)
All subprocess-based tests benefit automatically
No changes needed in individual test files
Helps both CI and local Windows development

References

Python docs:subprocess.CREATE_NO_WINDOW

PR checklist

• "closes[Bug]: Random timeout failures in CI #30851" is in the body of the PR description tolink the related issue
• new and changed code istested
• Plotting related features are demonstrated in anexample
• New Features andAPI Changes are noted with adirective and release note
• Documentation complies withgeneral anddocstring guidelines