Remove the back-compat alias shim for the truncate helpers

The underscore-prefixed _DEFAULT_MAX_RAW_MESSAGE / _cap_raw_message aliases
were kept only to ease migration to the now-public names; nothing imports
them, so drop the shim module and its test. The public DEFAULT_MAX_RAW_MESSAGE
and cap_raw_message remain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: antoineleclair