what circuitry and construction is required for safe signal levels
The simple solution is to transmit into a dummy load (read, a 50 ohm resistor from the parts drawer) set near the receiver.
If that doesn't provide enough coupling, you can build an attenuator of any value you need with some resistors. Search for "T pad attenuator".
Find the safe input power for the receiver from the datasheet. If you don't have a datasheet, then you can also look at typical transmitter powers and path loss for your application and determine a "typical" value that way.
Keep in mind more is not better. With increasing power, receiver distortion increases, which is not good if you are trying to measure distortion. Too little power and you are below the noise floor. Somewhere in the middle is ideal.
If your transmitter is battery powered, or powered by a DC isolated power supply, then you are already set. It's possible your receiver is already DC isolated.
If you don't want to chance it, then a 1:1 transformer is simple enough, though its specific construction will depend a lot on the frequency of interest.
A series capacitor is also an option, making a high-pass filter with the load impedance. Just make the capacitor large enough that the cutoff of this high-pass filter is well below your transmitter frequency.
Use coax if you want shielding.
ESD protection if needed
Probably not needed. Any receiver designed to be connected to an antenna will be sufficiently robust already.
impedance match if needed
Since you will have a lot of attenuation, the impedance of the attenuator will dominate the impedance seen by either side.
It may be the case that your transmitter needs to see a higher impedance than the conventional 50 ohms to remain within its current output specifications. The simplest solution is to use an attenuator of whatever impedance the transmitter needs to see. This means the receiver won't see a 50 ohm source, but all that means is you're getting less than the maximum possible power out of the source. Since you're building an attenuator that's not a problem. There is such a thing as a resistive T match which can present a difference impedance to each side, though in this application it would be of little practical benefit.
Though keep in mind the load impedance can affect the transmitter's output. Unless your antenna is also a matched load across all frequencies, you may not get a realistic measurement this way. This is especially true if you are spewing noise from a GPIO pin into a random wire with zero filtering whatsoever.