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/config/__init__.py

@@ -0,0 +1,8 @@
+"""Configuration models and loaders for the Stirling AI service."""
+
+from .settings import AppSettings, load_settings
+
+__all__ = [
+    "AppSettings",
+    "load_settings",
+]

engine/src/stirling/config/settings.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+from functools import lru_cache
+from pathlib import Path
+
+from dotenv import load_dotenv
+from pydantic import Field
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+ENGINE_ROOT = Path(__file__).resolve().parents[3]
+ENV_FILE = ENGINE_ROOT / ".env"
+
+
+class AppSettings(BaseSettings):
+    model_config = SettingsConfigDict(env_file=ENV_FILE, extra="ignore", populate_by_name=True)
+
+    smart_model_name: str = Field(validation_alias="STIRLING_SMART_MODEL")
+    fast_model_name: str = Field(validation_alias="STIRLING_FAST_MODEL")
+    smart_model_max_tokens: int = Field(validation_alias="STIRLING_SMART_MODEL_MAX_TOKENS")
+    fast_model_max_tokens: int = Field(validation_alias="STIRLING_FAST_MODEL_MAX_TOKENS")
+
+
+@lru_cache(maxsize=1)
+def load_settings() -> AppSettings:
+    load_dotenv(ENV_FILE)
+    return AppSettings.model_validate({})