feat: Add support for external knowledge directory via CREWAI_KNOWLEDGE_FILE_DIR

- Add get_knowledge_directory() utility function following CREWAI_STORAGE_DIR pattern
- Update BaseFileKnowledgeSource, CrewDoclingSource, and ExcelKnowledgeSource to use new function
- Add comprehensive tests for utility function and knowledge source integration
- Maintain backward compatibility with default 'knowledge' directory
- Add proper error handling for non-existent directories

Fixes #3519

Co-Authored-By: João <joao@crewai.com>

tests/utilities/test_knowledge_directory.py

@@ -0,0 +1,63 @@
+import os
+import tempfile
+from pathlib import Path
+import pytest
+from crewai.utilities.paths import get_knowledge_directory
+
+
+class TestKnowledgeDirectory:
+    def test_default_knowledge_directory(self):
+        """Test that default knowledge directory is returned when env var not set."""
+        if "CREWAI_KNOWLEDGE_FILE_DIR" in os.environ:
+            del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+        
+        result = get_knowledge_directory()
+        assert result == "knowledge"
+
+    def test_custom_knowledge_directory(self):
+        """Test that custom directory is returned when env var is set."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = temp_dir
+            try:
+                result = get_knowledge_directory()
+                assert result == temp_dir
+            finally:
+                del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+
+    def test_invalid_knowledge_directory(self):
+        """Test that ValueError is raised for non-existent directory."""
+        invalid_dir = "/path/that/does/not/exist"
+        os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = invalid_dir
+        try:
+            with pytest.raises(ValueError, match="Knowledge directory does not exist"):
+                get_knowledge_directory()
+        finally:
+            del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+
+    def test_relative_path_knowledge_directory(self):
+        """Test that relative paths work correctly."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            sub_dir = Path(temp_dir) / "knowledge_files"
+            sub_dir.mkdir()
+            
+            original_cwd = os.getcwd()
+            try:
+                os.chdir(temp_dir)
+                os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = "knowledge_files"
+                
+                result = get_knowledge_directory()
+                assert result == str(sub_dir)
+            finally:
+                os.chdir(original_cwd)
+                if "CREWAI_KNOWLEDGE_FILE_DIR" in os.environ:
+                    del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+
+    def test_absolute_path_knowledge_directory(self):
+        """Test that absolute paths work correctly."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = temp_dir
+            try:
+                result = get_knowledge_directory()
+                assert result == temp_dir
+            finally:
+                del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]