Add mock config server fixture

tests/conftest.py

@@ -1,11 +1,13 @@
 import importlib
+import json
 import os
 from collections.abc import AsyncGenerator
 from pathlib import Path
 from types import ModuleType
 from unittest.mock import patch
 
 import pytest
+from daq_config_server.converters.models import ConfigModel
 from ophyd_async.core import init_devices, set_mock_value
 
 from conftest import mock_attributes_table
@@ -88,3 +90,33 @@ async def baton_in_commissioning_mode() -> AsyncGenerator[Baton]:
     set_mock_value(baton.commissioning, True)
     yield baton
     set_commissioning_signal(None)
+
+
+def _fake_config_server_get_file_contents(
+    filepath: str | Path,
+    desired_return_type: type[str] | type[dict] | ConfigModel = str,
+    reset_cached_result: bool = True,
+):
+    filepath = Path(filepath)
+    # Minimal logic required for unit tests
+    with filepath.open("r") as f:
+        contents = f.read()
+        print(contents)
+        if desired_return_type is str:
+            return contents
+        elif desired_return_type is dict:
+            print("return type is dict")
+            return json.loads(contents)
+        elif issubclass(desired_return_type, ConfigModel):  # type: ignore
+            return desired_return_type.model_validate(json.loads(contents))
+
+
+@pytest.fixture(autouse=True)
+def mock_config_server():
+    # Don't actually talk to central service during unit tests, and reset caches between test
+
+    with patch(
+        "daq_config_server.client.ConfigServer.get_file_contents",
+        side_effect=_fake_config_server_get_file_contents,
+    ):
+        yield