infra: fake model invoke callback prior to yielding token (#18286)

## PR title
core[patch]: Invoke callback prior to yielding

## PR message
Description: Invoke on_llm_new_token callback prior to yielding token in
_stream and _astream methods.
Issue: https://github.com/langchain-ai/langchain/issues/16913
Dependencies: None
Twitter handle: None

libs/core/tests/unit_tests/fake/chat_model.py

@@ -225,9 +225,9 @@ class GenericFakeChatModel(BaseChatModel):
 
             for token in content_chunks:
                 chunk = ChatGenerationChunk(message=AIMessageChunk(content=token))
-                yield chunk
                 if run_manager:
                     run_manager.on_llm_new_token(token, chunk=chunk)
+                yield chunk
 
         if message.additional_kwargs:
             for key, value in message.additional_kwargs.items():
@@ -247,12 +247,12 @@ class GenericFakeChatModel(BaseChatModel):
                                         },
                                     )
                                 )
-                                yield chunk
                                 if run_manager:
                                     run_manager.on_llm_new_token(
                                         "",
                                         chunk=chunk,  # No token for function call
                                     )
+                                yield chunk
                         else:
                             chunk = ChatGenerationChunk(
                                 message=AIMessageChunk(
@@ -260,24 +260,24 @@ class GenericFakeChatModel(BaseChatModel):
                                     additional_kwargs={"function_call": {fkey: fvalue}},
                                 )
                             )
-                            yield chunk
                             if run_manager:
                                 run_manager.on_llm_new_token(
                                     "",
                                     chunk=chunk,  # No token for function call
                                 )
+                            yield chunk
                 else:
                     chunk = ChatGenerationChunk(
                         message=AIMessageChunk(
                             content="", additional_kwargs={key: value}
                         )
                     )
-                    yield chunk
                     if run_manager:
                         run_manager.on_llm_new_token(
                             "",
                             chunk=chunk,  # No token for function call
                         )
+                    yield chunk
 
     async def _astream(
         self,

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

@@ -396,14 +396,6 @@ async def test_event_stream_with_simple_chain() -> None:
             "run_id": "",
             "tags": ["my_chain", "my_model", "seq:step:2"],
         },
-        {
-            "data": {"chunk": AIMessageChunk(content="hello")},
-            "event": "on_chain_stream",
-            "metadata": {"foo": "bar"},
-            "name": "my_chain",
-            "run_id": "",
-            "tags": ["my_chain"],
-        },
         {
             "data": {"chunk": AIMessageChunk(content="hello")},
             "event": "on_chat_model_stream",
@@ -413,7 +405,7 @@ async def test_event_stream_with_simple_chain() -> None:
             "tags": ["my_chain", "my_model", "seq:step:2"],
         },
         {
-            "data": {"chunk": AIMessageChunk(content=" ")},
+            "data": {"chunk": AIMessageChunk(content="hello")},
             "event": "on_chain_stream",
             "metadata": {"foo": "bar"},
             "name": "my_chain",
@@ -429,7 +421,7 @@ async def test_event_stream_with_simple_chain() -> None:
             "tags": ["my_chain", "my_model", "seq:step:2"],
         },
         {
-            "data": {"chunk": AIMessageChunk(content="world!")},
+            "data": {"chunk": AIMessageChunk(content=" ")},
             "event": "on_chain_stream",
             "metadata": {"foo": "bar"},
             "name": "my_chain",
@@ -444,6 +436,14 @@ async def test_event_stream_with_simple_chain() -> None:
             "run_id": "",
             "tags": ["my_chain", "my_model", "seq:step:2"],
         },
+        {
+            "data": {"chunk": AIMessageChunk(content="world!")},
+            "event": "on_chain_stream",
+            "metadata": {"foo": "bar"},
+            "name": "my_chain",
+            "run_id": "",
+            "tags": ["my_chain"],
+        },
         {
             "data": {
                 "input": {