[red-knot] remove CallOutcome::Cast variant (#15461)

## Summary

Simplification follow-up to #15413.

There's no need to have a dedicated `CallOutcome` variant for every
known function, it's only necessary if the special-cased behavior of the
known function includes emitting extra diagnostics. For `typing.cast`,
there's no such need; we can use the regular `Callable` outcome variant,
and update the return type according to the cast. (This is the same way
we already handle `len`.)

One reason to avoid proliferating unnecessary `CallOutcome` variants is
that currently we have to explicitly add emitting call-binding
diagnostics, for each outcome variant. So we were previously wrongly
silencing any binding diagnostics on calls to `typing.cast`. Fixing this
revealed a separate bug, that we were emitting a bogus error anytime
more than one keyword argument mapped to a `**kwargs` parameter. So this
PR also adds test and fix for that bug.

## Test Plan

Existing `cast` tests pass unchanged, added new test for `**kwargs` bug.

crates/red_knot_python_semantic/resources/mdtest/call/function.md

@@ -169,6 +169,15 @@ def f(*args: int) -> int:
 reveal_type(f(1, 2, 3))  # revealed: int
 ```
 
+### Multiple keyword arguments map to keyword variadic parameter
+
+```py
+def f(**kwargs: int) -> int:
+    return 1
+
+reveal_type(f(foo=1, bar=2))  # revealed: int
+```
+
 ## Missing arguments
 
 ### No defaults or variadic