I thought solution is only if substance disassociates into its molecules, while particles would be dispersion or colloidal suspension.
Well yes this is the convention. Beyond semantics, though, who is to say the concentration of lone metal atoms at all times is exactly zero? If you consider the nucleation and growth process of nanoparticles, there is (as far as I know) still some disagreement whether nucleation occurs first by reduction to metal atoms, followed by coalescence, or coalescence of hydrated ions followed by reduction. Probably depends on the reaction conditions, but the point is: there are probably lone metal atoms "dissolved" in solution at some point, even if only in a transitional state. So no, I do not agree that metal cannot be dissolved in water as a neutral element.
I have learned that in chemistry as soon as you say "never" or "cannot", it doesn't take long to be proven wrong.