fix(vscode-ext): force matplotlib Agg backend more aggressively

Use force=True, switch_backend(), and close("all") after execution
to prevent plot windows from appearing during Python value resolution.

book/vscode-ext/src/providers/qmdPythonValueResolver.ts

@@ -111,12 +111,19 @@ function buildResolverScript(pythonSource: string, varNames: string[]): string {
   // We also suppress plt.show() so it's a no-op.
   const lines = [
     'import sys, json, os',
+    '# Force non-interactive backend BEFORE any matplotlib import.',
+    '# Setting the env var ensures even lazy imports pick up Agg.',
     'os.environ["MPLBACKEND"] = "Agg"',
+    'os.environ["MATPLOTLIB_BACKEND"] = "Agg"',
     'try:',
     '    import matplotlib',
-    '    matplotlib.use("Agg")',
+    '    matplotlib.use("Agg", force=True)',
     '    import matplotlib.pyplot as _plt',
+    '    _plt.switch_backend("Agg")',
     '    _plt.show = lambda *a, **k: None',
+    '    _plt.pause = lambda *a, **k: None',
+    '    # Close any figures that might have been created during import',
+    '    _plt.close("all")',
     'except ImportError:',
     '    pass',
     '',
@@ -133,6 +140,13 @@ function buildResolverScript(pythonSource: string, varNames: string[]): string {
     '# Execute the QMD Python blocks',
     pythonSource,
     '',
+    '# Close any figures created during execution',
+    'try:',
+    '    import matplotlib.pyplot as _plt2',
+    '    _plt2.close("all")',
+    'except Exception:',
+    '    pass',
+    '',
     '# Collect variable values',
     '_results = {}',
     captures,