Object index improvements (#12391)

docs/docs/examples/agent/agent_builder.ipynb

docs/docs/examples/agent/multi_document_agents-v1.ipynb

docs/docs/examples/agent/multi_document_agents.ipynb

docs/docs/examples/agent/openai_agent_retrieval.ipynb

docs/docs/examples/objects/object_index.ipynb

...cs/examples/query_engine/RetrieverRouterQueryEngine.ipynb

docs/docs/module_guides/deploying/agents/usage_pattern.md

llama-index-core/llama_index/core/objects/base.py

...-index-core/llama_index/core/objects/base_node_mapping.py

llama-index-core/llama_index/core/objects/fn_node_mapping.py

...index-core/llama_index/core/objects/table_node_mapping.py

...-index-core/llama_index/core/objects/tool_node_mapping.py

llama-index-core/llama_index/core/objects/utils.py

llama-index-core/tests/objects/test_base.py