[Internal] Make model serving unit tests a little more hermetic (#906)

## What changes are proposed in this pull request?

This PR is a no-op that makes the model serving unit tests a little more
hermetic. The solution is not perfect and we will have to consider
better ways to enable hermetic tests in the future (which might involve
a refactoring of the `Config` object in SDK-Mod).

## How is this tested?

N/A

NO_CHANGELOG=true

Author: renaudhartert-db

databricks/sdk/errors/base.py

@@ -7,7 +7,6 @@
 
 
 class ErrorDetail:
-
     def __init__(
         self,
         type: str = None,

tests/test_model_serving_auth.py

@@ -162,6 +162,12 @@ def test_model_serving_auth_refresh(monkeypatch, mocker):
 
 
 def test_agent_user_credentials(monkeypatch, mocker):
+    # Guarantee that the tests defaults to env variables rather than config file.
+    #
+    # TODO: this is hacky and we should find a better way to tell the config
+    # that it should not read from the config file.
+    monkeypatch.setenv("DATABRICKS_CONFIG_FILE", "x")
+
     monkeypatch.setenv("IS_IN_DB_MODEL_SERVING_ENV", "true")
     monkeypatch.setenv("DB_MODEL_SERVING_HOST_URL", "x")
     monkeypatch.setattr(
@@ -205,4 +211,4 @@ def test_agent_user_credentials_in_non_model_serving_environments(monkeypatch):
     headers = cfg.authenticate()
 
     assert cfg.host == "https://x"
-    assert headers.get("Authorization") == f"Bearer token"
+    assert headers.get("Authorization") == "Bearer token"