chore(langchain): improve typing of agent decorators

[cbornet]

• Fix typing of async functions for @wrap_model_call and @wrap_tool_call
• Use type aliases instead of protocols. This is better regarding open/close principle: this way the user can use any names for the function parameters, only the position is important.
• Remove useless generics
• Use cast when mypy can't narrow the type (iscoroutinefunction doesn't narrow the callables, and the types in the unions are too complex for a TypeGuard to work)

related: #34583 #34249