test: add unit tests and fixtures

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/fixtures/chatgpt_conversation.json

@@ -0,0 +1,57 @@
+{
+  "id": "chatgpt-conv-001",
+  "title": "Python Async Tutorial",
+  "create_time": 1704067200.0,
+  "update_time": 1704153600.0,
+  "project_title": "Learning Python",
+  "mapping": {
+    "node-root": {
+      "id": "node-root",
+      "parent": null,
+      "children": ["node-1"],
+      "message": null
+    },
+    "node-1": {
+      "id": "node-1",
+      "parent": "node-root",
+      "children": ["node-2"],
+      "message": {
+        "id": "node-1",
+        "author": {"role": "user"},
+        "create_time": 1704067200.0,
+        "content": {
+          "content_type": "text",
+          "parts": ["How does async/await work in Python?"]
+        }
+      }
+    },
+    "node-2": {
+      "id": "node-2",
+      "parent": "node-1",
+      "children": ["node-3"],
+      "message": {
+        "id": "node-2",
+        "author": {"role": "assistant"},
+        "create_time": 1704067210.0,
+        "content": {
+          "content_type": "text",
+          "parts": ["Async/await in Python allows you to write concurrent code using coroutines.\n\n```python\nimport asyncio\n\nasync def main():\n    await asyncio.sleep(1)\n    print('Done!')\n\nasyncio.run(main())\n```"]
+        }
+      }
+    },
+    "node-3": {
+      "id": "node-3",
+      "parent": "node-2",
+      "children": [],
+      "message": {
+        "id": "node-3",
+        "author": {"role": "user"},
+        "create_time": 1704067300.0,
+        "content": {
+          "content_type": "image_asset_pointer",
+          "parts": [{"content_type": "image_asset_pointer", "asset_pointer": "file://some-image"}]
+        }
+      }
+    }
+  }
+}