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/tests/test_pdf_question_agent.py

@@ -0,0 +1,79 @@
+from __future__ import annotations
+
+import pytest
+
+from stirling.agents import PdfQuestionAgent
+from stirling.config import AppSettings
+from stirling.contracts import (
+    PdfQuestionAnswerResponse,
+    PdfQuestionNeedTextResponse,
+    PdfQuestionNotFoundResponse,
+    PdfQuestionRequest,
+)
+from stirling.services import build_runtime
+
+
+class StubPdfQuestionAgent(PdfQuestionAgent):
+    def __init__(self, response: PdfQuestionAnswerResponse | PdfQuestionNotFoundResponse) -> None:
+        super().__init__(build_runtime(build_test_settings()))
+        self.response = response
+
+    async def _run_answer_agent(
+        self,
+        request: PdfQuestionRequest,
+    ) -> PdfQuestionAnswerResponse | PdfQuestionNotFoundResponse:
+        return self.response
+
+
+def build_test_settings() -> AppSettings:
+    return AppSettings(
+        smart_model_name="test",
+        fast_model_name="test",
+        smart_model_max_tokens=8192,
+        fast_model_max_tokens=2048,
+    )
+
+
+@pytest.mark.anyio
+async def test_pdf_question_agent_requires_extracted_text() -> None:
+    agent = PdfQuestionAgent(build_runtime(build_test_settings()))
+
+    response = await agent.handle(PdfQuestionRequest(question="What is the total?", extracted_text=""))
+
+    assert isinstance(response, PdfQuestionNeedTextResponse)
+
+
+@pytest.mark.anyio
+async def test_pdf_question_agent_returns_grounded_answer() -> None:
+    agent = StubPdfQuestionAgent(
+        PdfQuestionAnswerResponse(
+            answer="The invoice total is 120.00.",
+            evidence=["Invoice total: 120.00"],
+        )
+    )
+
+    response = await agent.handle(
+        PdfQuestionRequest(
+            question="What is the total?",
+            extracted_text="Invoice total: 120.00",
+            file_name="invoice.pdf",
+        )
+    )
+
+    assert isinstance(response, PdfQuestionAnswerResponse)
+    assert response.answer == "The invoice total is 120.00."
+
+
+@pytest.mark.anyio
+async def test_pdf_question_agent_returns_not_found_when_text_is_insufficient() -> None:
+    agent = StubPdfQuestionAgent(PdfQuestionNotFoundResponse(reason="The answer is not present in the text."))
+
+    response = await agent.handle(
+        PdfQuestionRequest(
+            question="What is the total?",
+            extracted_text="This page contains only a shipping address.",
+            file_name="invoice.pdf",
+        )
+    )
+
+    assert isinstance(response, PdfQuestionNotFoundResponse)