Raise exception of blend_func issue

[emmanuel-ferdman]

PR Summary

This small PR raises a ValueError exception when blend_func got a wrong tuple.

[Copilot]

Pull Request Overview

A concise description of the purpose of the PR, followed by summarized bullets of changes

• Raises a ValueError when blend_func receives a tuple of invalid length.
• Adds the missing raise keyword for the error case in blend_func.

Comments suppressed due to low confidence (1)

arcade/gl/backends/opengl/context.py:143

• Add a unit test to verify that providing an invalid tuple length to blend_func raises ValueError.

def blend_func(self, value: Tuple[int, int] | Tuple[int, int, int, int]):