core[patch]: Deduplicate of callback handlers in merge_configs

[eyurtsev]

This PR adds deduplication of callback handlers in merge_configs.

Fix for this issue: #22227

The issue appears when the code is:

1. running python >=3.11
2. invokes a runnable from within a runnable
3. binds the callbacks to the child runnable from the parent runnable using with_config

In this case, the same callbacks end up appearing twice: (1) the first time from with_config, (2) the second time with langchain automatically propagating them on behalf of the user.

Prior to this PR this will emit duplicate events:

@tool
async def get_items(question: str, callbacks: Callbacks):  # <--- Accept callbacks
    """Ask question"""
    template = ChatPromptTemplate.from_messages(
        [
            (
                "human",
                "'{question}"
            )
        ]
    )
    chain = template | chat_model.with_config(
        {
            "callbacks": callbacks,  # <-- Propagate callbacks
        }
    )
    return await chain.ainvoke({"question": question})

Prior to this PR this will work work correctly (no duplicate events):

@tool
async def get_items(question: str, callbacks: Callbacks):  # <--- Accept callbacks
    """Ask question"""
    template = ChatPromptTemplate.from_messages(
        [
            (
                "human",
                "'{question}"
            )
        ]
    )
    chain = template | chat_model
    return await chain.ainvoke({"question": question}, {"callbacks": callbacks})

This will also work (as long as the user is using python >= 3.11) -- as langchain will automatically propagate callbacks

@tool
async def get_items(question: str,):  
    """Ask question"""
    template = ChatPromptTemplate.from_messages(
        [
            (
                "human",
                "'{question}"
            )
        ]
    )
    chain = template | chat_model
    return await chain.ainvoke({"question": question})

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[eyurtsev]

Investigating source of issue for: #22227