If a Warlock selects Pact of the Chain, they gain the ability to summon a pseudodragon familiar with Find Familiar.
Pseudodragons have a sidebar in the Monster Manual entitled Variant: Pseudodragon Familiar. It reads:
VARIANT: PSEUDODRAGON FAMILIAR
Some pseudodragons are willing to serve spellcasters as a familiar. Such pseudodragons have the following trait. Familiar. The pseudodragon can serve another creature as a familiar, forming a magic, telepathic bond with that willing companion. While the two are bonded, the companion can sense what the pseudodragon senses as long as they are within 1 mile of each other. While the pseudodragon is within 10 feet of its companion, the companion shares the pseudodragon's Magic Resistance trait. At any time and for any reason, the pseudodragon can end its service as a familiar, ending the telepathic bond.
This describes a variant ruling where a spellcaster can befriend a pseudodragon and gain the Magic Resistance trait. However, because a warlock summons the pseudodragon directly, it's not clear if this would apply to their situation.
If a Warlock summons a pseudodragon familiar through pact of the chain, does the Warlock also benefit from the pseudodragon's Magic Resistance trait?