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/contracts/pdf_edit.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from typing import Annotated, Literal
+
+from pydantic import Field
+
+from stirling.models import ApiModel
+
+from .common import ToolOperationStep
+
+
+class PdfEditRequest(ApiModel):
+    user_message: str
+    conversation_id: str | None = None
+    file_names: list[str] = Field(default_factory=list)
+
+
+class EditPlanResponse(ApiModel):
+    outcome: Literal["plan"] = "plan"
+    summary: str
+    rationale: str | None = None
+    steps: list[ToolOperationStep]
+
+
+class EditClarificationRequest(ApiModel):
+    outcome: Literal["need_clarification"] = "need_clarification"
+    question: str
+    reason: str
+
+
+class EditCannotDoResponse(ApiModel):
+    outcome: Literal["cannot_do"] = "cannot_do"
+    reason: str
+
+
+PdfEditResponse = Annotated[
+    EditPlanResponse | EditClarificationRequest | EditCannotDoResponse,
+    Field(discriminator="outcome"),
+]