core[patch]: Deduplicate of callback handlers in merge_configs (#22478)

This PR adds deduplication of callback handlers in merge_configs.

Fix for this issue:
https://github.com/langchain-ai/langchain/issues/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:

```python
@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):

```python
@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

```python
@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})
```

libs/core/langchain_core/runnables/config.py

@@ -305,12 +305,12 @@ def merge_configs(*configs: Optional[RunnableConfig]) -> RunnableConfig:
                         base["callbacks"] = mngr
                     else:
                         # base_callbacks is also a manager
-                        base["callbacks"] = base_callbacks.__class__(
+
+                        manager = base_callbacks.__class__(
                             parent_run_id=base_callbacks.parent_run_id
                             or these_callbacks.parent_run_id,
-                            handlers=base_callbacks.handlers + these_callbacks.handlers,
-                            inheritable_handlers=base_callbacks.inheritable_handlers
-                            + these_callbacks.inheritable_handlers,
+                            handlers=[],
+                            inheritable_handlers=[],
                             tags=list(set(base_callbacks.tags + these_callbacks.tags)),
                             inheritable_tags=list(
                                 set(
@@ -323,6 +323,20 @@ def merge_configs(*configs: Optional[RunnableConfig]) -> RunnableConfig:
                                 **these_callbacks.metadata,
                             },
                         )
+
+                        handlers = base_callbacks.handlers + these_callbacks.handlers
+                        inheritable_handlers = (
+                            base_callbacks.inheritable_handlers
+                            + these_callbacks.inheritable_handlers
+                        )
+
+                        for handler in handlers:
+                            manager.add_handler(handler)
+
+                        for handler in inheritable_handlers:
+                            manager.add_handler(handler, inherit=True)
+
+                        base["callbacks"] = manager
             else:
                 base[key] = config[key] or base.get(key)  # type: ignore
     return base

libs/core/tests/unit_tests/runnables/test_runnable_events_v2.py

@@ -1876,3 +1876,34 @@ async def test_runnable_generator() -> None:
             "tags": [],
         },
     ]
+
+
+async def test_with_explicit_config() -> None:
+    """Test astream events with explicit callbacks being passed."""
+    infinite_cycle = cycle([AIMessage(content="hello world", id="ai3")])
+    model = GenericFakeChatModel(messages=infinite_cycle)
+
+    @tool
+    async def say_hello(query: str, callbacks: Callbacks) -> BaseMessage:
+        """Use this tool to look up which items are in the given place."""
+
+        @RunnableLambda
+        def passthrough_to_trigger_issue(x: str) -> str:
+            """Add passthrough to trigger issue."""
+            return x
+
+        chain = passthrough_to_trigger_issue | model.with_config(
+            {"tags": ["hello"], "callbacks": callbacks}
+        )
+
+        return await chain.ainvoke(query)
+
+    events = await _collect_events(
+        say_hello.astream_events("meow", version="v2")  # type: ignore
+    )
+
+    assert [
+        event["data"]["chunk"].content
+        for event in events
+        if event["event"] == "on_chat_model_stream"
+    ] == ["hello", " ", "world"]