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

demos/affinity_testing/config.yaml

@@ -0,0 +1,31 @@
+version: v0.4.0
+
+listeners:
+  - type: model
+    name: model_listener
+    port: 12000
+
+model_providers:
+  - model: openai/gpt-5.2
+    access_key: $OPENAI_API_KEY
+    default: true
+
+  - model: openai/gpt-5.2-chat-latest
+    access_key: $OPENAI_API_KEY
+
+routing_preferences:
+  - name: complex_reasoning
+    description: complex reasoning tasks, multi-step analysis, or detailed explanations
+    models:
+      - openai/gpt-5.2
+      - openai/gpt-5.2-chat-latest
+
+  - name: code_generation
+    description: generating new code, writing functions, or creating boilerplate
+    models:
+      - openai/gpt-5.2-chat-latest
+      - openai/gpt-5.2
+
+routing:
+  session_ttl_seconds: 600
+  session_max_entries: 1000