chore(langchain): remove generic from FakeToolCallingModel

[cbornet]

• Making FakeToolCallingModel generic on its structured_response doesn't help anywhere in typing.
• There are more than 120 references of FakeToolCallingModel in the code where you get error: Need type annotation for "model" [var-annotated] because mypy can't resolve the generic type (we don't see them atm because they are in files temporarily excluded from mypy checking). We would need to explicitly type them to FakeToolCallingModel[Any]