what is the poison in chinaberry?
The toxin is Meliatoxin, in a class of toxins called tetranortriterpenes.
I found this here.  Bold emphasis mine.
https://wagwalking.com/condition/chinaberry-tree-poisoning

What is Chinaberry Poisoning? The chinaberry tree is a member of the
mahogany family that is native to Australia, China, and India. These
deciduous trees sprout delicate light purple flowers in the spring,
which develop into yellow, berry-like fruits in the summer. They were
transported to North America in the early 1800’s as an ornamental
plant, but has escaped into the environment and is considered an
invasive species in several southern states. It produces a potent
insecticide which utilizes a tetranortriterpene neurotoxin known as
meliatoxin. This insecticide is found throughout the plant’s bark,
leaves, and flowers but is found in the highest concentrations in the
ripe fruit. If you suspect that your pet has consumed any of this
plant, contact your veterinarian immediately. The chinaberry tree, or
Melia azedarach, is a deciduous tree that produces a potent
insecticide which utilizes a neurotoxin that can prove detrimental to
your pet if ingested. Symptoms of Chinaberry Poisoning in Dogs
Symptoms of chinaberry tree poisoning occur most often from eating the
ripe fruit, however, the bark, leaves, and flowers also contain
smaller amounts of the toxic substances. Symptoms usually begin within
just two to four hours from ingestion. Abdominal pain Blood in stool
Colic Constipation Death Depression Diarrhea         Excessive
drooling Heart attack Hyperactivity Lack of coordination Loss of
appetite Muscle rigidity Seizures Slowed heart rate Stupor Vomiting
Weakness Types
The scientific name for the chinaberry tree is Melia
azedarach, and it belongs in the mahogany family.

From another source
http://www.sciencedirect.com/science/article/pii/0031942283830398

Toxic tetranortriterpenes of the fruit of Melia azedarach . .Peter &gt;B.Oelrichs. Michael W.Hill. Peter J.Vallely. John K.MacLeod. .Tadeusz &gt;F.Molinski
Show more https://doi.org/10.1016/0031-9422(83)83039-8Get rights and
content Abstract Four new tetranortriterpenes, meliatoxins A1, A2, B1
and B2 have been isolated and identified as toxic constituents of the
fruit of Melia azedarach L. var. australasica. Toxicity and
pathological results confirm that the meliatoxins are responsible for
most but not all of the symptoms resulting from the ingestion of whole
fruit.