fix(ci): harden publishing and validate examples

tools/test_prepare_pdf_sources.py

@@ -2,12 +2,24 @@ import tempfile
 import unittest
 from base64 import b64encode
 from pathlib import Path
-
-from tools.prepare_pdf_sources import normalize_markdown_asset_paths, prepare_pdf_sources
+import subprocess
+import sys
 
 
 class PreparePdfSourcesTest(unittest.TestCase):
+    def test_module_imports_with_running_python(self):
+        result = subprocess.run(
+            [sys.executable, "-c", "import tools.prepare_pdf_sources"],
+            cwd=Path(__file__).resolve().parents[1],
+            capture_output=True,
+            text=True,
+        )
+
+        self.assertEqual(result.returncode, 0, result.stderr)
+
     def test_normalizes_relative_markdown_and_html_image_paths(self):
+        from tools.prepare_pdf_sources import normalize_markdown_asset_paths
+
         with tempfile.TemporaryDirectory() as tmp:
             book_dir = Path(tmp)
             (book_dir / "_images").mkdir()
@@ -31,6 +43,8 @@ class PreparePdfSourcesTest(unittest.TestCase):
             self.assertEqual(count, 2)
 
     def test_prepares_temp_tree_without_mutating_source_markdown(self):
+        from tools.prepare_pdf_sources import prepare_pdf_sources
+
         with tempfile.TemporaryDirectory() as tmp:
             source = Path(tmp) / "source"
             target = Path(tmp) / "target"