fix: correct @langchain/openai version to ^0.5.0 (compatible with @langchain/core ^0.3.28)

Version 0.3.18 doesn't exist. Using context7 research found that:
- @langchain/openai@0.5.0 requires @langchain/core >=0.2.26 <0.4.0
- This is compatible with our current @langchain/core ^0.3.28

Prevents yesterday's 8-hour debugging session from recurring.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: ScientiaCapital

package.json

@@ -28,7 +28,7 @@
     "@huggingface/hub": "^2.6.5",
     "@langchain/core": "^0.3.28",
     "@langchain/langgraph": "^0.2.42",
-    "@langchain/openai": "^0.3.18",
+    "@langchain/openai": "^0.5.0",
     "langchain": "^0.3.11",
     "@opentelemetry/api": "^1.9.0",
     "@opentelemetry/exporter-prometheus": "^0.205.0",