`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

mypy test.py の例から始めて、UNBOUND_TYPEVAR 診断がどこで生成され、テストされているかを特定します。メッセージを要求された短い文言に更新し、その後、3 つの失敗する例が新しい診断を出力し、有効な例は変更されていないことを確認します。