Room interior wall not built straight

It's just stupid when you wanted to place something right-angled in the corner.

You don't even place a side table directly "in the corner."
And all larger furniture is anyway 5-6 cm away from the wall.
This also applies to a bed, as you want to hide the sheet at the edges. Watch out for edge and wall protection.

It is not uncommon in children's rooms.

Then I place the long side against the wall and the short side of a child's bed (1m) is only 3cm away from the wall.

But if I go to a furniture store with the plan and buy a cabinet that according to the plan would fit with 2.45m, and then order it, but the room on the other end is only 2.35m, that is definitely a defect. Or am I completely wrong?

Then have the carpenter come again and have her change the wall. It is a defect and you have to give her the opportunity to repair it.

By the way, you NEVER buy furniture based on the raw construction measurements in the plan, but according to the actual measurements. If only because several centimeters can be added for the plaster. Or because of surprises like in your case.

But if I go to a furniture store with the plan and buy a cabinet that according to the plan would fit with 2.45m and then order it, but the room at the other end is only 2.35m, that is definitely a defect. Or am I completely wrong?

Plans show the shell construction. The plaster is missing. And that still has tolerances. Order furniture only after measuring with finished plaster. Measure at several heights. Or plan buffers, for example with kitchens you often have finishing strips, which should be „on paper“ wide enough to still be able to become narrower after measuring post-plaster.

But if I go to a furniture store with the plan and buy a wardrobe that should fit according to the plan with 2.45m and then order it, but the room is only 2.35m at the other end, that is definitely a defect. Or am I completely wrong?

Yes, you don’t do that. RBM doesn’t work, only actual measurements.