feat(langchain): use decorators for jumps instead (#33179)
The old `before_model_jump_to` classvar approach was quite clunky, this
is nicer imo and easier to document. Also moving from `jump_to` to
`can_jump_to` which is more idiomatic.
Before:
```py
class MyMiddleware(AgentMiddleware):
before_model_jump_to: ClassVar[list[JumpTo]] = ["end"]
def before_model(state, runtime) -> dict[str, Any]:
return {"jump_to": "end"}
```
After
```py
class MyMiddleware(AgentMiddleware):
@hook_config(can_jump_to=["end"])
def before_model(state, runtime) -> dict[str, Any]:
return {"jump_to": "end"}
```
