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/api/routes/__init__.py

@@ -0,0 +1,13 @@
+from .agent_drafts import router as agent_draft_router
+from .execution import router as execution_router
+from .orchestrator import router as orchestrator_router
+from .pdf_edit import router as pdf_edit_router
+from .pdf_questions import router as pdf_question_router
+
+__all__ = [
+    "agent_draft_router",
+    "execution_router",
+    "orchestrator_router",
+    "pdf_edit_router",
+    "pdf_question_router",
+]