Fix CI test failures: restore missing classes and fix imports

- Restored CompletionResponse and CompletionResponseStreamChunk classes in inference.py
- Fixed SamplingParams import in test_resolver.py
- Fixed openai_compat.py imports
- Added missing pytest configuration files
- Fixed registry error messages
- All fixes needed to make CI tests pass

tests/conftest.py

@@ -0,0 +1,8 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+# Global pytest plugins configuration
+pytest_plugins = ["tests.integration.fixtures.common"]