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/app.py

@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+from contextlib import asynccontextmanager
+from typing import Annotated
+
+from fastapi import Depends, FastAPI
+
+from stirling.agents import ExecutionPlanningAgent, OrchestratorAgent, PdfEditAgent, PdfQuestionAgent, UserSpecAgent
+from stirling.api.routes import (
+    agent_draft_router,
+    execution_router,
+    orchestrator_router,
+    pdf_edit_router,
+    pdf_question_router,
+)
+from stirling.config import AppSettings, load_settings
+from stirling.contracts import HealthResponse
+from stirling.services import build_runtime
+
+
+def _load_startup_settings(fast_api: FastAPI) -> AppSettings:
+    override = fast_api.dependency_overrides.get(load_settings)
+    if override is not None:
+        return override()
+    return load_settings()
+
+
+@asynccontextmanager
+async def lifespan(fast_api: FastAPI):
+    # Load env vars on startup so we can immediately crash if required env vars aren't set
+    settings = _load_startup_settings(fast_api)
+    runtime = build_runtime(settings)
+    fast_api.state.settings = settings
+    fast_api.state.runtime = runtime
+    fast_api.state.orchestrator_agent = OrchestratorAgent(runtime)
+    fast_api.state.pdf_edit_agent = PdfEditAgent(runtime)
+    fast_api.state.pdf_question_agent = PdfQuestionAgent(runtime)
+    fast_api.state.user_spec_agent = UserSpecAgent(runtime)
+    fast_api.state.execution_planning_agent = ExecutionPlanningAgent(runtime)
+    yield
+
+
+app = FastAPI(title="Stirling AI Engine", lifespan=lifespan, version="0.1.0")
+app.include_router(orchestrator_router)
+app.include_router(pdf_edit_router)
+app.include_router(pdf_question_router)
+app.include_router(agent_draft_router)
+app.include_router(execution_router)
+
+
+@app.get("/health", response_model=HealthResponse)
+async def healthcheck(settings: Annotated[AppSettings, Depends(load_settings)]) -> HealthResponse:
+    return HealthResponse(
+        status="ok",
+        smart_model=settings.smart_model_name,
+        fast_model=settings.fast_model_name,
+    )