feat(langchain): enhance MergerRetriever to support mixed retriever

[khushmanvar]

Enhance MergerRetriever to support mixed retriever types

• Updated the retrievers attribute in MergerRetriever to accept both BaseRetriever and RetrieverOutputLike types.
• Modified the _get_relevant_documents method to handle invocation for both types of retrievers.
• Added comprehensive unit tests for MergerRetriever to validate functionality with mixed retriever types, including async support and integration with history-aware retrievers.

Fixes #33184

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[eyurtsev]

Would RetrieverLike work? (Since it has as input a str?)

RetrieverLike = Runnable[RetrieverInput, RetrieverOutput]