Rangers expertise

Does the energy cost reduction of a ranger's expertise attribute reduce the cost of casting attack spells? Specifically mesmer spells?

No.

It only affects those skills whose descriptor is "attack skill"

There is a monk skill, Smite, that may be affected for example if it is still an "attack skill", but most of the casters use "spells" which are unaffected. Holy Strike is another monk skill that might be, as it is not a spell according to the description I read but is a skill as well, though doesn't specifically say "attack skill", so I doubt it...

I was under the impression that it only affected ranger skills.

Nope, it affects Preparations, Traps and Attack Skills - since Warriors have attack skills it affects those - however, Warrior attack skills tend not to be that expensive, and they also rely heavily on adrenaline skills. Ranger/Warriors have the advantage of being able to use many more attack skills based on energy as they recover energy faster, pay less for the skills and have a larger pool of energy. By the description it should also affect the Monk skill "Smite" as I mentioned above, but I have never checked.