`UNBOUND_TYPEVAR` error message should be more direct and shorter

*Notes:

• mypy test.py
• mypy 1.19.1
• Python 3.14.0
• Related comment

The last 3 examples get the same indirect and long error messages of UNBOUND_TYPEVAR as shown below:

*Notes:

• A parameter is a variable defined in a function header to receive an argument.
• An argument is a value passed to a function parameter.

A function returning TypeVar should receive at least one argument containing the same TypeVar

With T for x:

def func[T](x: T) -> T:
    return x
# No error

Without T for x:

def func[T](x) -> T:
    return x
# error: A function returning TypeVar should receive at least one argument containing the same TypeVar

With no parameters:

from typing import cast

def func[T]() -> T:
    return cast(T, 'Hello')
# error: A function returning TypeVar should receive at least one argument containing the same TypeVar

With T1 for x and T2 for return type:

def func[T1, T2](x: T1) -> T2:
    return x
# error: A function returning TypeVar should receive at least one argument containing the same TypeVar

So, the same indirect and long error messages of UNBOUND_TYPEVAR should be changed:

From:

A function returning TypeVar should receive at least one argument containing the same TypeVar

To more direct and shorter one below:

A function returning TypeVar should have at least one same TypeVar parameter

Beginne mit den Beispielen in mypy test.py und ermittle, wo die Diagnose UNBOUND_TYPEVAR erzeugt und getestet wird. Aktualisiere die Meldung auf die gewünschte kürzere Form und überprüfe anschließend, dass die drei fehlschlagenden Beispiele die neue Diagnose erzeugen, ohne das gültige Beispiel zu ändern.