Great question — but according to the wiki, Mendoza did sign his rookie contract. There's no indication he's unsigned.
Per the Fernando Mendoza page and Las Vegas Raiders page, the No. 1 overall pick signed a four-year deal worth $54.566 million (about $5.7M more than last year's No. 1 pick Cam Ward), including a $36.144 million signing bonus that's fully guaranteed. The cap hit is prorated across the four years at roughly $9.9M against the 2026 cap (from the 2026 NFL Draft: How much will each first-round pick be paid? source).
If you saw a report suggesting he's unsigned, it may be outdated or confused with another player — for example, Jacoby Brissett (Cardinals) was in a real contract holdout situation this offseason, but Mendoza's deal has been locked in since the draft.