Mending has an instantaneous duration
The duration of Mending is instantaneous so its effect is not ongoing. Since there is no ongoing magic a mended brick would not be detected by Detect Magic, could not be opened by Dispel Magic, and would not be broken by an antimagic field.
There are some spells, such as Ceremony, or Heroes Feast that are instantaneous but can create a prolonged effect. These spells, however, have clearly defined durations as a part of the spells text.
Other spells, such as Leomund's Secret Chest, and Goodberry do not have a clearly defined end to their duration1 but have some mechanism that degrade their magic power over time.
Functionally there is a bit of a grey area with regards to instantaneous spells with enduring effects but as a rule unless the nature of the effect is unambiguously magical (ie. being able to conjure a chest from the ethereal plane) it is best considered nonmagical.
1:The berries from Goodberry never disappear they just lose their potency