Redesign Python AI engine (#5991)

# Description of Changes
Redesign the Python AI engine to be properly agentic and make use of
`pydantic-ai` instead of `langchain` for correctness and ergonomics.
This should be a good foundation for us to build our AI engine on going
forwards.

engine/src/stirling/__init__.py

@@ -0,0 +1,7 @@
+"""Stirling AI engine package."""
+
+from .api.app import app
+
+__all__ = [
+    "app",
+]