Re-enable animation constraints on animation verifier

Animation constraints were disabled about 5 or so years ago because of a viewer crash bug regarding chain length / constraint targets. This bug has since been fixed as far as I can tell (At least, my repro animation doesn't crash the Linden viewer or mainstream TPVs).

Given the fact that invalid chain lengths / constraint targets are now ignored by the viewer, there isn't really a need to verify the chain length anymore.

It would be really nice if we could have the ability to add constraints in animations again!

This is basically just IK for bones, think feet automatically aligning to the ground's curvature, or shape independant animations that can put the hands on the head, without it looking like the hands are floating above or inside the head.

(This feature ALREADY EXISTS in the viewer. In fact, default animations use this mechanism. It was disabled on the server side animation verifier a few years back due to a bug that I found that has since been fixed.)

https://secondlife.canny.io/admin/board/feature-requests/p/re-enable-animation-constraints-on-animation-verifier

No source file or test is named. Start by locating the server-side animation verifier and reviewing the linked Canny feature request alongside its constraint validation logic. Done means valid animation constraints are accepted again while invalid chain lengths or targets remain safely ignored without viewer crashes.