Update external memory tests to handle optional ChromaDB dependency

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

tests/memory/external_memory_test.py

@@ -169,6 +169,15 @@ def test_crew_external_memory_reset(mem_type, crew_with_external_memory):
 def test_crew_external_memory_save_with_memory_flag(
     mem_method, crew_with_external_memory
 ):
+    try:
+        import chromadb
+        HAS_CHROMADB = True
+    except ImportError:
+        HAS_CHROMADB = False
+        
+    if not HAS_CHROMADB:
+        pytest.skip("ChromaDB is required for this test")
+        
     with patch(
         f"crewai.memory.external.external_memory.ExternalMemory.{mem_method}"
     ) as mock_method: