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

Benchmark test

The local slowdown is primarily a side effect of security software, while the CI speedup comes from reducing operating system overhead.

Why it's slower locally
On a local Windows machine, antivirus and Windows Defender often perform more aggressive heuristic scans on "windowless" processes. Because these processes have no UI, they are flagged as more suspicious, and the extra scanning time per process adds up during benchmarks.

Why it will run faster in CI
CI environments (like GitHub Actions) are headless and highly resource-constrained, making them sensitive to OS-level bottlenecks rather than antivirus scans:

Eliminates conhost.exe: Every console window requires the OS to spawn a conhost.exe process. In CI, bypassing this reduces the total memory and CPU load.

Preserves Desktop Heap: Creating many windows can exhaust the Windows "Desktop Heap," which leads to the unpredictable "random" timeouts seen in the issue.

Reduces Resource Contention: Without the need to initialize UI handles for every test subprocess, the system can allocate more resources to the actual test logic.

PR checklist

• "closes [Bug]: Random timeout failures in CI #30851" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines