SAN FRANCISCO — The wildfires that have devastated Northern California this month caused at least $1 billion in damage to insured property, officials said Thursday, as authorities increased the count of homes and other buildings destroyed to nearly 7,000. Both numbers were expected to rise as crews continued assessing areas scorched by the blazes that killed 42 people, a total that makes it the deadliest series of fires in state history. State Insurance Commissioner Dave Jones said the preliminary dollar valuation of losses came from claims filed with the eight largest insurance companies in the affected areas and did not include uninsured property. The loss total was expected to climb “probably dramatically so,” Jones told reporters, making it likely the fires also would become the costliest in California’s history.