feat(demos): add affinity testing demo for model pinning

demos/affinity_testing/README.md

@@ -0,0 +1,46 @@
+# Affinity Testing (OpenAI SDK)
+
+Quick demo to verify model affinity pinning using standard OpenAI SDK calls against Plano.
+
+## 1) Start Plano with affinity config
+
+```bash
+export OPENAI_API_KEY=<your-key>
+
+planoai up demos/affinity_testing/config.yaml
+```
+
+`config.yaml` enables affinity cache settings:
+
+```yaml
+routing:
+  session_ttl_seconds: 600
+  session_max_entries: 1000
+```
+
+## 2) Run the demo script
+
+```bash
+python demos/affinity_testing/demo.py
+```
+
+The script uses this exact SDK pattern:
+
+```python
+from openai import OpenAI
+import uuid
+
+client = OpenAI(base_url="http://localhost:12000/v1", api_key="EMPTY")
+affinity_id = str(uuid.uuid4())
+
+response = client.chat.completions.create(
+    model="gpt-5.2",
+    messages=messages,
+    extra_headers={"X-Model-Affinity": affinity_id},
+)
+```
+
+## Expected behavior
+
+- Call 1 and call 2 share the same affinity ID and should stay on the same selected model.
+- Call 3 uses a new affinity ID and should be free to route independently.