Return UnknownTargetError for VuMark generation with non-existent target

[adamtheturtle]

When a VuMark instance generation request is made via POST /targets/<target-id>/instances with a target ID that does not exist in the database, the mock should return an UnknownTargetError response.

Currently, the request results in a ConnectionError because no route matches.

See the xfail test in vws-cli: VWS-Python/vws-cli#1968

[adamtheturtle]

This is now working. The validate_target_id_exists validator correctly handles the /targets/<target-id>/instances path pattern, and a passing test was added in #2985 confirming that an unknown target ID returns UnknownTarget (HTTP 404).