Fix intermittent test failures on Windows CI due to Tcl/Tk initialization race condition (#558)

Plot tests on Windows CI were failing intermittently with
`_tkinter.TclError: Can't find a usable init.tcl`, causing flaky test
runs that would pass on retry. This was a race condition where uv's
Python extraction on Windows didn't complete before matplotlib tried to
initialize Tkinter.

This fix sets `MPLBACKEND=Agg` in the test workflow, which tells
matplotlib to use the non-interactive Agg backend instead of Tkinter.
This is standard practice for CI environments and eliminates the Tcl
dependency entirely. Plot functionality remains fully tested—the Agg
backend generates static images instead of interactive windows.

Author: tompollard

.github/workflows/test.yml

@@ -31,6 +31,8 @@ jobs:
         if: startsWith(matrix.os, 'ubuntu')
         run: sudo apt-get install -y libsndfile1
       - name: Run tests
+        env:
+          MPLBACKEND: Agg
         run: uv run --with ${{ matrix.numpy }} --extra dev pytest
 
   test-deb11-i386: