Photodiode + electronics solution:
An optical sensor with good spectral bandwidth and 125C max temperature rating can be obtained by using a photodiode with separate electronics. A very good performance unit is mentioned below by way of example, but lower cost solutions with performance that meets you requirements are available.
The ODD-#W-2 photodiode has a 125 degrees C maximum operating temperature and 400-100 nm spectral response. The response varies with wavelength but can be linearised in software (or with hardware) and spectral range could be limited with optical filters if desired. These cost $27.41 in unit quantity, in stock at Digikey - but far lower cost diodes are also available if unit cost matters.
BPW24R $4.50/1 125C max opn, UV to IR response.
BPW20RF $6.30/1 125 C max opn, UV to IR
A selection of in-stock photodiodes with 125C max operating temperature are listed here
Optical sensors including integral electronics:
This Digikey search offers optical sensors with 105C or 125C max temperature . A possibly major concern is that these are usually targeted at IR sensing and the spectral response may be too narrow, depending on your application.
Wider spectral range sensor: This -40C to 105C operating range sensor claims "near human eye spectral response" and is in stock at Digikey in unit quantities.
Sensitivity is high (100 lux max) but a suitable ND filter would allow this to be extended.
In lieu of high-temperature-rated sensors you might consider:
Reflecting the light to a sensor outside the enclosed space, and calibrate it for the reflection and transmission losses. Mirror can be perhaps 5% loss , or under 1% if front silvered. Transmission through window glass depends on material and wavelength but is usually under 10%. Calibration should allow this method to work adequately for most applications.
A heatsink of small area extending either through the glass or embedded in the frame so minimal thermal flow occurs apart from via the sensor.
Fibre-optics for conveying light to an external sensor.
Piping in cooling air (or a recirculating feed) may be possible without overly perturbing the in window space energy balance. A temperature sensor monitoring the light sensor would allow minimal air (or other fluid) flow.
Excellent TI/Burr Brown app note
PHOTODIODE MONITORING WITH OP AMPS
Another Ti/Burr Brown Photo diode app note - you do not need to use their nice but no doubt expensive IC.
DESIGNING PHOTODIODE AMPLIFIER CIRCUITS WITH OPA128
Sharp Photodiode/Phototransistor Application Circuit
Microchip - AN951 Amplifying High-Impedance Sensors – Photodiode Example
Hamatsu some ideas
Hofoo. Who? App notes and data galore. May even be useful.
AD - somewhat OTT but provides ideas
2 MHz Bandwidth PIN Photodiode Preamp with Dark Current Compensation