What is the ruling on wishing for death and refusing medical treatment due to despair of life, and is that considered a rejection of Allah's blessings and a lack of patience with tribulation, especially with the fear of Allah's retribution for not thanking Him and rejecting life?

It is not permissible to wish for death out of impatience or discontent with God's decrees. If a person is compelled to do so, they should say: "O Allah, keep me alive as long as life is better for me, and take my soul if death is better for me." This is because a believer is hoped to increase in good deeds, and a non-believer might repent and seek forgiveness. It is forbidden to abstain from medicine if its abandonment leads to destruction. Indeed, seeking treatment may be obligatory if it is known to ensure the preservation of life. Therefore, a Muslim must be patient with God's divine decree and destiny, for all that God decrees for a believer is good for them. God may test His servant with afflictions to elevate them to a status they would not achieve through their deeds alone.