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/common.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from typing import Literal
+
+from pydantic import model_validator
+
+from stirling.models import OPERATIONS, ApiModel, OperationId, ParamToolModel
+
+
+class ConversationMessage(ApiModel):
+    role: str
+    content: str
+
+
+class PdfTextSelection(ApiModel):
+    page_number: int | None = None
+    text: str
+
+
+class ToolOperationStep(ApiModel):
+    kind: Literal["tool"] = "tool"
+    tool: OperationId
+    parameters: ParamToolModel
+
+    @model_validator(mode="after")
+    def validate_tool_parameter_pairing(self) -> ToolOperationStep:
+        expected_type = OPERATIONS[self.tool]
+        if not isinstance(self.parameters, expected_type):
+            actual_type = type(self.parameters).__name__
+            expected_type_name = expected_type.__name__
+            raise ValueError(f"Parameters for tool {self.tool.value} must be {expected_type_name}, got {actual_type}.")
+        return self