E8-T1: Fix DefaultModel to match DeploymentName in appsettings

Co-authored-by: dylan-mccarthy <[email protected]>

Author: Copilot

src/Node.Runtime/appsettings.json

@@ -30,7 +30,7 @@
     "ValidateCertificateChain": true
   },
   "AgentRuntime": {
-    "DefaultModel": "gpt-5-mini",
+    "DefaultModel": "gpt-4o-mini",
     "DefaultTemperature": 0.7,
     "MaxTokens": 4000,
     "MaxDurationSeconds": 60,