Mahāsamādhi in Buddhism?

miccha-samadhi (wrong/defiled concentration). And to think the mind is in the body is also miccha-samadhi if so perceived.

the act of consciously and intentionally leaving one's body.

I haven't ran across any Buddhist suttas that mention the above as a practice of the Buddhism's samadhi. In Buddhism, they don't concern much with "maha" samadhi. Instead they focus more on Samma-Samadhi/Right-Concentration, which is a link in the Noble Eightfold Path. Refer to this dictionary's definition for further details from Buddhism's perspective.

I don't think there is mahasamadi in buddhism but there is mind made body. This is what the Buddha said about mind made body in DN 2.

With his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability, he directs and inclines it to creating a mind-made body. From this body he creates another body, endowed with form, made of the mind, complete in all its parts, not inferior in its faculties. Just as if a man were to draw a reed from its sheath. The thought would occur to him: 'This is the sheath, this is the reed. The sheath is one thing, the reed another, but the reed has been drawn out from the sheath.' Or as if a man were to draw a sword from its scabbard. The thought would occur to him: 'This is the sword, this is the scabbard. The sword is one thing, the scabbard another, but the sword has been drawn out from the scabbard.' Or as if a man were to pull a snake out from its slough. The thought would occur to him: 'This is the snake, this is the slough. The snake is one thing, the slough another, but the snake has been pulled out from the slough.' In the same way — with his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability, the monk directs and inclines it to creating a mind-made body. From this body he creates another body, endowed with form, made of the mind, complete in all its parts, not inferior in its faculties.

"This, too, great king, is a fruit of the contemplative life, visible here and now, more excellent than the previous ones and more sublime.

Yes. The concept of relinquishing the will to live can be found in Buddhism.

From DN 16:

And the Blessed One said: "Whosoever, Ananda, has developed, practiced, employed, strengthened, maintained, scrutinized, and brought to perfection the four constituents of psychic power could, if he so desired, remain throughout a world-period or until the end of it. The Tathagata, Ananda, has done so. Therefore the Tathagata could, if he so desired, remain throughout a world-period or until the end of it."

(And later)

When this was said, the Blessed One spoke to Mara, the Evil One, saying: "Do not trouble yourself, Evil One. Before long the Parinibbana of the Tathagata will come about. Three months hence the Tathagata will utterly pass away."

And at the Capala shrine the Blessed One thus mindfully and clearly comprehending renounced his will to live on. And upon the Lord's renouncing his will to live on, there came a tremendous earthquake, dreadful and astonishing, and thunder rolled across the heavens.

Commentary (Sister Vajira & Francis Story):

Kappam va tittheyya kappavasesam va. Comy. takes kappa not as "world-period" or "aeon," but as ayu-kappa, "life span," and explains avasesa (usually "remainder") by "in excess." Comy.: "He may stay alive completing the life span pertaining to men at the given time. (Sub. Comy.: the maximum life span.) Kappavasesa: 'in excess' (atireka), i.e., more or less above the hundred years said to be the normally highest life expectation."

Among the numerous meanings of the word kappa, there is, in fact, that of time in general (kala) and not only the duration of an aeon; but the meaning "life span" seems to have been ascribed to it only in this passage. Also, the meaning "in excess" for avasesa (usually "remainder") is unusual.

The four constituents of psychic power (iddhipada) are concentration due to zeal, energy, purity of mind, and investigation.

The Anandajoti translation states:

Whoever has developed, Ānanda, made much of, carried on, established, maintained, augmented, and properly instigated the Four Paths to Power, could, if he wanted, Ānanda, remain for the lifespan or for what is left of the lifespan. The Realised One has developed, Ānanda, made much of, carried on, established, maintained, augmented, and properly instigated the Four Paths to Power. If he wanted, Ānanda, the Realised One could remain for the lifespan or for what is left of the lifespan.”