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_stirling_contracts.py

@@ -0,0 +1,78 @@
+from collections.abc import Iterator
+
+import pytest
+
+from stirling.config import AppSettings, load_settings
+from stirling.contracts import (
+    AgentExecutionRequest,
+    AgentSpec,
+    AgentSpecStep,
+    EditPlanResponse,
+    ExecutionContext,
+    OrchestratorRequest,
+    PdfQuestionAnswerResponse,
+    ToolOperationStep,
+)
+from stirling.models.tool_models import OperationId, RotateParams
+
+
+def test_orchestrator_request_accepts_user_message() -> None:
+    request = OrchestratorRequest(user_message="Rotate the PDF")
+
+    assert request.user_message == "Rotate the PDF"
+
+
+def test_agent_execution_request_uses_typed_agent_spec() -> None:
+    steps: list[AgentSpecStep] = [
+        ToolOperationStep(
+            tool=OperationId.ROTATE,
+            parameters=RotateParams(angle=90),
+        )
+    ]
+    request = AgentExecutionRequest(
+        agent_spec=AgentSpec(
+            name="Invoice cleanup",
+            description="Normalise inbound invoices",
+            objective="Prepare uploads for accounting review",
+            steps=steps,
+        ),
+        current_step_index=0,
+        execution_context=ExecutionContext(input_files=["invoice.pdf"]),
+    )
+
+    assert request.agent_spec.steps[0].kind == "tool"
+
+
+def test_edit_plan_response_has_typed_steps() -> None:
+    steps = [ToolOperationStep(tool=OperationId.ROTATE, parameters=RotateParams(angle=90))]
+    response = EditPlanResponse(
+        summary="Rotate the input PDF by 90 degrees.",
+        steps=steps,
+    )
+
+    assert response.steps[0].tool == OperationId.ROTATE
+
+
+def test_pdf_question_answer_defaults_evidence_list() -> None:
+    response = PdfQuestionAnswerResponse(answer="The invoice total is 120.00")
+
+    assert response.evidence == []
+
+
+@pytest.fixture(autouse=True)
+def clear_settings_cache() -> Iterator[None]:
+    load_settings.cache_clear()
+    yield
+    load_settings.cache_clear()
+
+
+def test_app_settings_accepts_model_configuration() -> None:
+    settings = AppSettings(
+        smart_model_name="claude-sonnet-4-5-20250929",
+        fast_model_name="claude-haiku-4-5-20251001",
+        smart_model_max_tokens=8192,
+        fast_model_max_tokens=2048,
+    )
+
+    assert settings.smart_model_name
+    assert settings.fast_model_max_tokens == 2048