Add environment variable to control category filtering

OCIO color spaces contain a categories attribute that allows config authors and applications to filter the list of color spaces shown in a given menu based on its function (e.g. working spaces, textures). The ColorSpaceMenuHelper class provides app developers an easy way to use this functionality.

There is currently a OCIO_USER_CATEGORIES_ENVVAR that allows users to specify categories to use with application menus. It would be nice if there was one or more additional env. vars to control menu filtering. For example, it might be useful to be able to turn off filtering being done by an application.

Discussed at the 2025-09-29 TSC meeting.

Start by locating ColorSpaceMenuHelper and the handling of OCIO_USER_CATEGORIES_ENVVAR. Review the 2025-09-29 TSC discussion to determine which additional environment variable behavior is wanted, including whether application filtering can be disabled. Done means implementing the agreed behavior with corresponding coverage.

Written by the indexing model from the issue text.