Is the husband's saying to his wife via text message "I am an unbeliever" - with his awareness of its meaning and his request for her to marry someone else - considered a dissolution of the marriage?

Whoever describes himself as a disbeliever (kafir), a Jew, a Christian, or an atheist, willingly and out of his own free choice, has disbelieved and apostatized from Islam.

The effect of this on the marriage differs: If it was before consummation (ad-dukhool), his wife is immediately separated from him. If it was after consummation, the wife is obliged to abstain from him, and the separation (furqah) is contingent upon the expiration of the waiting period (iddah). If he returns to Islam before the expiration of the iddah, the marital bond remains. But if he persists in his apostasy until the iddah expires, then the marriage is deemed annulled from the time of apostasy. The wife, after her iddah expires and he has not returned to Islam, is permitted to marry another.