Show a clear message when PyMuPDF is missing

tests/test_pdf_runtime.py

@@ -0,0 +1,24 @@
+import builtins
+
+import pytest
+
+from src.pdf_runtime import PDF_VIEWER_PYMUPDF_MISSING, load_pymupdf_for_pdf_viewer
+
+
+def test_pdf_viewer_dependency_error_is_user_actionable(monkeypatch):
+    real_import = builtins.__import__
+
+    def fake_import(name, *args, **kwargs):
+        if name == "fitz":
+            raise ImportError("No module named fitz")
+        return real_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(builtins, "__import__", fake_import)
+
+    with pytest.raises(RuntimeError) as exc:
+        load_pymupdf_for_pdf_viewer()
+
+    message = str(exc.value)
+    assert message == PDF_VIEWER_PYMUPDF_MISSING
+    assert "requirements-optional.txt" in message
+    assert "PyMuPDF" in message