feat(retriever): Add bm25 strategy and fix related init/retrieve bugs

[zhaoyizhu6-pixel]

• examples/run_lightmem_bm25.py:

  • Added a new example script to test the bm25 strategy end-to-end.

• src/lightmem/configs/base.py:

  • Added "bm25" to the Literal types for index_strategy and retrieve_strategy to allow the new config.

• src/lightmem/configs/memory_manager/base.py:

  • Fixed a ValidationError by changing configs: Optional[dict] to Optional[Any] to match the validator's logic.

• src/lightmem/configs/retriever/bm25.py:

  • Fixed a ValidationError by adding the required collection_name and path fields to BM25Config.

• src/lightmem/memory/lightmem.py:

  • Replaced the retrieve method, which was hardcoded for "embedding", with a new version that correctly checks the retrieve_strategy and calls the context_retriever for "bm25".
  • Updated offline_update to correctly call index() when index_strategy="bm25".
  • Fixed multiple AttributeErrors in __init__ related to compressor, model_name, and text_embedder to allow non-embedding strategies to initialize correctly.