core[patch]: Fix FileCallbackHandler name resolution, Fixes #29941 (#29942)

- **Description:** Same changes as #26593 but for FileCallbackHandler
- **Issue:**  Fixes #29941
- **Dependencies:** None
- **Twitter handle:** None

- [x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

libs/core/langchain_core/callbacks/file.py

@@ -47,9 +47,15 @@ class FileCallbackHandler(BaseCallbackHandler):
             inputs (Dict[str, Any]): The inputs to the chain.
             **kwargs (Any): Additional keyword arguments.
         """
-        class_name = serialized.get("name", serialized.get("id", ["<unknown>"])[-1])
+        if "name" in kwargs:
+            name = kwargs["name"]
+        else:
+            if serialized:
+                name = serialized.get("name", serialized.get("id", ["<unknown>"])[-1])
+            else:
+                name = "<unknown>"
         print_text(
-            f"\n\n\033[1m> Entering new {class_name} chain...\033[0m",
+            f"\n\n\033[1m> Entering new {name} chain...\033[0m",
             end="\n",
             file=self.file,
         )

libs/langchain/tests/unit_tests/callbacks/test_file.py

@@ -0,0 +1,45 @@
+import pathlib
+from typing import Any, Dict, List, Optional
+
+import pytest
+
+from langchain.callbacks import FileCallbackHandler
+from langchain.chains.base import CallbackManagerForChainRun, Chain
+
+
+class FakeChain(Chain):
+    """Fake chain class for testing purposes."""
+
+    be_correct: bool = True
+    the_input_keys: List[str] = ["foo"]
+    the_output_keys: List[str] = ["bar"]
+
+    @property
+    def input_keys(self) -> List[str]:
+        """Input keys."""
+        return self.the_input_keys
+
+    @property
+    def output_keys(self) -> List[str]:
+        """Output key of bar."""
+        return self.the_output_keys
+
+    def _call(
+        self,
+        inputs: Dict[str, str],
+        run_manager: Optional[CallbackManagerForChainRun] = None,
+    ) -> Dict[str, str]:
+        return {"bar": "bar"}
+
+
+def test_filecallback(capsys: pytest.CaptureFixture, tmp_path: pathlib.Path) -> Any:
+    """Test the file callback handler."""
+    p = tmp_path / "output.log"
+    handler = FileCallbackHandler(str(p))
+    chain_test = FakeChain(callbacks=[handler])
+    chain_test.invoke({"foo": "bar"})
+    # Assert the output is as expected
+    assert p.read_text() == (
+        "\n\n\x1b[1m> Entering new FakeChain "
+        "chain...\x1b[0m\n\n\x1b[1m> Finished chain.\x1b[0m\n"
+    )