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/execution.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+from typing import Annotated
+
+from fastapi import APIRouter, Depends
+
+from stirling.agents import ExecutionPlanningAgent
+from stirling.api.dependencies import get_execution_planning_agent
+from stirling.contracts import AgentExecutionRequest, NextExecutionAction
+
+router = APIRouter(prefix="/api/v1/agents", tags=["agents"])
+
+
+@router.post("/next-action", response_model=NextExecutionAction)
+async def next_action(
+    request: AgentExecutionRequest,
+    agent: Annotated[ExecutionPlanningAgent, Depends(get_execution_planning_agent)],
+) -> NextExecutionAction:
+    return await agent.next_action(request)