Hi and welcome
Cannot understand why your in warranty as its normally three years only, unless you took out extended warranty, or you bought the car recently and got a one year warranty included.
Anyway a lot of things are not covered by warranty (read the small print), things like expendable parts, like brakes, oil changes, A/C, etc, and I think your fan has done very well reaching over 100,000 miles. Now if the car had only done say 30,000 miles and the fan/motor was worn, you would be well within in your rights to ask for the dealer to pay. But 100,000 miles no chance, that fan is just worn out. You could try asking dealer, but chances are the answer is no.
The fan needs changing, so cheapest method is buy one off ebay and fit it yourself, its an easy bolt on job. Done a few in my time. Just make sure you get the right radiator fan to fit your engine type/size. Or get an independent garage to fit one for you.