Move 'get_project_base_directory' to common directory (#10940)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

test/unit_test/common/test_file_utils.py

@@ -0,0 +1,123 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+import os
+import pytest
+from unittest.mock import patch
+from common import file_utils
+from common.file_utils import get_project_base_directory
+
+
+class TestGetProjectBaseDirectory:
+    """Test cases for get_project_base_directory function"""
+
+    def test_returns_project_base_when_no_args(self):
+        """Test that function returns project base directory when no arguments provided"""
+        result = get_project_base_directory()
+
+        assert result is not None
+        assert isinstance(result, str)
+        assert os.path.isabs(result)  # Should return absolute path
+
+    def test_returns_path_with_single_argument(self):
+        """Test that function joins project base with single additional path component"""
+        result = get_project_base_directory("subfolder")
+
+        assert result is not None
+        assert "subfolder" in result
+        assert result.endswith("subfolder")
+
+    def test_returns_path_with_multiple_arguments(self):
+        """Test that function joins project base with multiple path components"""
+        result = get_project_base_directory("folder1", "folder2", "file.txt")
+
+        assert result is not None
+        assert "folder1" in result
+        assert "folder2" in result
+        assert "file.txt" in result
+        assert os.path.basename(result) == "file.txt"
+
+    def test_uses_environment_variable_when_available(self):
+        """Test that function uses RAG_PROJECT_BASE environment variable when set"""
+        test_path = "/custom/project/path"
+
+        file_utils.PROJECT_BASE = test_path
+
+        result = get_project_base_directory()
+        assert result == test_path
+
+    def test_calculates_default_path_when_no_env_vars(self):
+        """Test that function calculates default path when no environment variables are set"""
+        with patch.dict(os.environ, {}, clear=True):  # Clear all environment variables
+            # Reset the global variable to force re-initialization
+
+            result = get_project_base_directory()
+
+            # Should return a valid absolute path
+            assert result is not None
+            assert os.path.isabs(result)
+            assert os.path.basename(result) != ""  # Should not be root directory
+
+    def test_caches_project_base_value(self):
+        """Test that PROJECT_BASE is cached after first calculation"""
+        # Reset the global variable
+
+        # First call should calculate the value
+        first_result = get_project_base_directory()
+
+        # Store the current value
+        cached_value = file_utils.PROJECT_BASE
+
+        # Second call should use cached value
+        second_result = get_project_base_directory()
+
+        assert first_result == second_result
+        assert file_utils.PROJECT_BASE == cached_value
+
+    def test_path_components_joined_correctly(self):
+        """Test that path components are properly joined with the base directory"""
+        base_path = get_project_base_directory()
+        expected_path = os.path.join(base_path, "data", "files", "document.txt")
+
+        result = get_project_base_directory("data", "files", "document.txt")
+
+        assert result == expected_path
+
+    def test_handles_empty_string_arguments(self):
+        """Test that function handles empty string arguments correctly"""
+        result = get_project_base_directory("")
+
+        # Should still return a valid path (base directory)
+        assert result is not None
+        assert os.path.isabs(result)
+
+
+# Parameterized tests for different path combinations
+@pytest.mark.parametrize("path_args,expected_suffix", [
+    ((), ""),  # No additional arguments
+    (("src",), "src"),
+    (("data", "models"), os.path.join("data", "models")),
+    (("config", "app", "settings.json"), os.path.join("config", "app", "settings.json")),
+])
+def test_various_path_combinations(path_args, expected_suffix):
+    """Test various combinations of path arguments"""
+    base_path = get_project_base_directory()
+    result = get_project_base_directory(*path_args)
+
+    if expected_suffix:
+        assert result.endswith(expected_suffix)
+    else:
+        assert result == base_path