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/pdf_edit.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+from typing import Annotated
+
+from fastapi import APIRouter, Depends
+
+from stirling.agents import PdfEditAgent
+from stirling.api.dependencies import get_pdf_edit_agent
+from stirling.contracts import PdfEditRequest, PdfEditResponse
+
+router = APIRouter(prefix="/api/v1/pdf/edit", tags=["pdf-edit"])
+
+
+@router.post("", response_model=PdfEditResponse)
+async def pdf_edit(
+    request: PdfEditRequest,
+    agent: Annotated[PdfEditAgent, Depends(get_pdf_edit_agent)],
+) -> PdfEditResponse:
+    return await agent.handle(request)