test: add unit tests and fixtures

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

tests/fixtures/chatgpt_no_project.json

@@ -0,0 +1,43 @@
+{
+  "id": "chatgpt-conv-002",
+  "title": "Quick Question",
+  "create_time": 1706745600.0,
+  "update_time": 1706745900.0,
+  "project_title": null,
+  "mapping": {
+    "node-root": {
+      "id": "node-root",
+      "parent": null,
+      "children": ["msg-1"],
+      "message": null
+    },
+    "msg-1": {
+      "id": "msg-1",
+      "parent": "node-root",
+      "children": ["msg-2"],
+      "message": {
+        "id": "msg-1",
+        "author": {"role": "user"},
+        "create_time": 1706745600.0,
+        "content": {
+          "content_type": "text",
+          "parts": ["What is the capital of France?"]
+        }
+      }
+    },
+    "msg-2": {
+      "id": "msg-2",
+      "parent": "msg-1",
+      "children": [],
+      "message": {
+        "id": "msg-2",
+        "author": {"role": "assistant"},
+        "create_time": 1706745610.0,
+        "content": {
+          "content_type": "text",
+          "parts": ["The capital of France is Paris."]
+        }
+      }
+    }
+  }
+}