Fix type inference for walrus operator across keyword arguments

[Vaishnavi-Iyer67]

Problem

Mypy fails to infer the type of a variable assigned via a named expression (:=)
when that variable is referenced in another keyword argument of the same call.

Although Python guarantees left-to-right evaluation order for function arguments,
mypy currently type-checks keyword arguments independently, leading to a false
positive has-type error.

Minimal example

def f(notifications: str, initial_reference: str) -> int:
    return 42

f(
    initial_reference=(request_reference := "abc"),
    notifications=request_reference,  # error: Cannot determine type
)