fix(agents): add default None values to RetrieverTools class attributes (#306)

Bare type annotations without default values do not create class
attributes in Python — they only populate __annotations__. This caused
hasattr(RetrieverTools, "install_retriever") to return False, breaking
TestRetrieverTools.test_class_attributes_structure.

Signed-off-by: Jack Luar <jluar@precisioninno.com>

Author: luarss

backend/src/agents/retriever_tools.py

@@ -21,22 +21,22 @@ def __init__(self) -> None:
 
     install_retriever: Optional[
         Union[EnsembleRetriever, ContextualCompressionRetriever]
-    ]
+    ] = None
     general_retriever: Optional[
         Union[EnsembleRetriever, ContextualCompressionRetriever]
-    ]
+    ] = None
     commands_retriever: Optional[
         Union[EnsembleRetriever, ContextualCompressionRetriever]
-    ]
+    ] = None
     errinfo_retriever: Optional[
         Union[EnsembleRetriever, ContextualCompressionRetriever]
-    ]
+    ] = None
     yosys_rtdocs_retriever: Optional[
         Union[EnsembleRetriever, ContextualCompressionRetriever]
-    ]
+    ] = None
     klayout_retriever: Optional[
         Union[EnsembleRetriever, ContextualCompressionRetriever]
-    ]
+    ] = None
     tool_descriptions: str = ""
 
     def initialize(