Who covers extermination and sealing costs
In California, the landlord generally pays for professional rodent extermination and for sealing entry points, because keeping the rental free of rodents is part of the landlord's habitability duty.
When rats or mice infest a unit or building, hiring and paying a licensed pest-control company is typically the landlord's responsibility, and so is repairing the holes rodents use to get in. Tenants are expected to store food properly and cooperate with treatment, but they usually should not have to pay for the infestation itself.
The picture can change if a tenant's serious neglect clearly created the problem, in which case some cost might fall on the tenant. But for the ordinary building infestation, the bill belongs with the landlord. If yours is trying to charge you for extermination that should be their responsibility, a free review can help you understand what is fair.
This is general information about California tenant rights, not legal advice. Every situation is different.