Picking a fruit or two off a tree whose branches are hanging over a public road is not uncommon, and is sometimes tolerated by owners, but it is not legal according to the letter of the law. The Code Civil says fruit still attached to a tree remain a part of it and (article 520) the ‘immoveable’ property of the owner.
On the other hand, article 673 clarifies that fruit that has dropped off the tree becomes the property of the owner of the land it falls on – in the case of a neighbour’s garden, the neighbour, or if it is a public road, it is fair game to any passer-by.
Collecting fallen fruit is known as glanage – this user-generated map gives suggestions for places you may find it: https://fallingfruit.org/
According to the charity France Nature Environnement, there is a law in Franche-Comté that says any crops left unharvested by owners after November 1 are considered abandoned and available to anyone.
The charity also says that as a general principle in France glanage of a few fallen fruits on fields and orchards by hand after the harvest and in daylight is traditionally tolerated on unenclosed land, unless there is a specific local law banning it.