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/services/__init__.py

@@ -0,0 +1,9 @@
+"""Shared services used by the Stirling AI runtime."""
+
+from .runtime import AppRuntime, build_model_settings, build_runtime
+
+__all__ = [
+    "AppRuntime",
+    "build_model_settings",
+    "build_runtime",
+]

engine/src/stirling/services/runtime.py

@@ -0,0 +1,51 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from pydantic_ai.models import Model, infer_model
+from pydantic_ai.settings import ModelSettings
+
+from stirling.config import AppSettings
+
+
+@dataclass(frozen=True)
+class AppRuntime:
+    settings: AppSettings
+    fast_model: Model
+    smart_model: Model
+
+    @property
+    def fast_model_settings(self) -> ModelSettings:
+        return build_model_settings(self.settings.fast_model_max_tokens)
+
+    @property
+    def smart_model_settings(self) -> ModelSettings:
+        return build_model_settings(self.settings.smart_model_max_tokens)
+
+
+def build_model_settings(max_tokens: int | None) -> ModelSettings:
+    model_settings: ModelSettings = {}
+    if max_tokens is not None:
+        model_settings["max_tokens"] = max_tokens
+    return model_settings
+
+
+def validate_structured_output_support(model: Model, model_name: str) -> None:
+    # Pydantic AI's dedicated test model does not advertise native structured output,
+    # but we still use it in unit tests as a non-production stand-in.
+    if model_name == "test":
+        return
+    if not model.profile.supports_json_schema_output:
+        raise ValueError(f"Unsupported model {model_name}. This model does not support structured outputs.")
+
+
+def build_runtime(settings: AppSettings) -> AppRuntime:
+    fast_model = infer_model(settings.fast_model_name)
+    smart_model = infer_model(settings.smart_model_name)
+    validate_structured_output_support(fast_model, settings.fast_model_name)
+    validate_structured_output_support(smart_model, settings.smart_model_name)
+    return AppRuntime(
+        settings=settings,
+        fast_model=fast_model,
+        smart_model=smart_model,
+    )