How are runway lights constructed so that they are not damaged by aircraft wheels, and can continue to function safely, to demanding specifications, under a wide variety of environmental conditions?
I presume you are asking about lights that are on the surface of the runway like centerline lights and touch down zone lights. The FAA has an AC that covers their design and installation which you can find here. Take a look at section 4.5 "In Pavement Light Fixture Testing". This section outlines some of the force requirements the light must be able to withstand. The FAA currently requires the light to be mounted in an L-868 base can which is imbedded in the runway during construction. There are also a bunch of size requirements laid out in section "3.4. Dimensional Requirements." that describe how the lights are to be built.
A runway edge light won't survive an encounter with an aircraft wheel. They're just heavy duty light fixtures on short posts. Heavy glass lenses, well sealed, with materials that can survive outside exposure and stand up to snow shot across by plows, etc.
If an airplane takes out a runway light, there will be much more excitement over the follow-on activities happening to the airplane compared than with runway light's fate.
There is no protection.
While dead heading in the copilot seat, the pilot of a Beech 18 cut a corner too tight and mowed a couple down (Kodiak Alaska). An animated discussion with the airport manager soon followed. Close inspection revealed the light posts slip into a base and are held with set screws, it broke and bent parts off the post and base.