Allow errors keyword argument in codecs _Encoder, _Decoder, and stream protocols - #16697

Fixes #16441.

In stdlib/codecs.pyi, _Encoder, _Decoder, _StreamReader, and _StreamWriter were annotated with trailing /, making all parameters (including errors: str = ...) positional-only.

However, codecs.Codec.encode, codecs.Codec.decode, and encoder/decoder implementations accept errors by keyword (e.g. codecs.lookup('utf-8').decode(b'potato', errors='replace')). This caused mypy to issue false positive errors:

error: Unexpected keyword argument "errors" for "__call__" of "_Decoder" [call-arg]

By placing the / after input / stream (def __call__(self, input: ReadableBuffer, /, errors: str = ...) -> tuple[str, int]: ...), the first argument remains positional-only for flexible parameter naming across implementations, while errors is accessible both positionally and by keyword.

Added type assertion tests in stdlib/@tests/test_cases/check_codecs.py covering keyword errors calls on info.decode and info.encode.