What kind of controller are you interfacing with?
If you switch the LEDs directly, what does the voltage rise up to when open circuit? Does the power supply have open circuit fault detection, is there anything to be aware of there? (It may be useful as an intended protection mechanism.) Does it not, and it grenades when left open? Or should it be treated as a true CC supply which is
shorted to set the zero-power state? (Real power supplies usually have output filter caps, so aren't CC on short time scales -- which is to say, a shunt switch would have to absorb the discharge current of those capacitors, and that has to be factored in the design.)
Would it be desirable to have the light output throttle down instead? Should it flash on and off naturally (temperature + hysteresis)? Or with a set delay? Or should it be latched? If latched, what resets it? Power cycle?
The absolute minimum implementation, assuming a well behaved power source, is simply the thermistor, with another bias resistor to make a voltage divider; a fixed voltage divider; a comparator between the two dividers, with hysteresis; and a MOSFET. That's it. The comparator does need some supply voltage itself, which may require an additional connection, or a regulator. Several of these components can be found together in a single chip, though I suspect you won't find a three-terminal, ready-to-go solution. (Protected MOSFETs are painfully close, but their temperature thresholds are probably too high.)
If these components cannot be fitted onto an MCPCB, well, layout issues are your responsibility; we certainly can't help you without any design files to work from.
If you require a complete solution, you will have to provide your existing design files, and a requirements document listing the restrictions you are working under. There are many contractors on this forum, available at reasonable rates. If you are expecting an utterly free solution, who knows, maybe someone will volunteer one. Eventually. But, eh, you get what you pay for, as they say.
Tim