docs(thesis): resolve review findings and verify examples

tests/docs/test_thesis_langgraph_example.py

@@ -0,0 +1,28 @@
+"""Exercise the comparison snippet when the optional LangGraph library exists."""
+
+from pathlib import Path
+
+import pytest
+
+
+@pytest.mark.parametrize(
+    ("text", "expected_report"),
+    [(" Notes ", "# Report\n\nNotes"), ("   ", "")],
+)
+def test_langgraph_comparison_routes_updated_state(
+    text: str, expected_report: str
+) -> None:
+    """Check the rendered example without making LangGraph a product dependency."""
+    pytest.importorskip("langgraph.graph")
+    manuscript = (
+        Path(__file__).resolve().parents[2]
+        / "docs/thesis/system-design-implementation.md"
+    ).read_text(encoding="utf-8")
+    section = manuscript.split("<!-- langgraph-comparison -->", 1)[1]
+    snippet = section.split("```python\n", 1)[1].split("```", 1)[0]
+    namespace = {}
+    # This is trusted repository prose, executed exactly as displayed.
+    exec(compile(snippet, "thesis-langgraph-example", "exec"), namespace)
+    result = namespace["graph"].invoke({"text": text, "report": ""})
+    assert result["text"] == text.strip()
+    assert result["report"] == expected_report